IIRC, the defender used to get the 5/10/15 defense bonuses not just when defending but all the time back in FE.
Unless there was a bug, the defense bonuses were only in effect while defending in FE. So no, it wasn't a trait that gave Defenders the ability to stay with poor armor longer.
Mages gained basically nothing from multi-classing whereas every other class gained a lot by it.
I think Mages gained hugely from multiclassing to Defender in FE - consider that by level 10 an extra +2 health per level meant that you had ~50% more health than you normally would have. It also gave you an extra 20 weight capacity, which meant that you could ignore picking up Strength or hoping for a backpack if you wanted your Mage to wear anything heavier than leather and still have a decent initiative level. That the other classes gained more from multiclasses doesn't mean that Mages gained next to nothing from it.
Mages also gained a decent amount from multiclassing to Assassin, because damage spells could get critical hits and an Assassin/Mage could have a reasonably high likelihood of attaining a critical. You think Horrific Wail from a level 15 Mage is good? Try Horrific Wail from a level 15 Assassin/Mage when ~half the targets suffer critical hits - even high-level late game units have trouble surviving that.
This isn't to say that other classes didn't lose out more when multiclassing was dropped.
@Joeball - I meant Knowledge, not Potential. The one that mages get that others don't. That should be tradeable just fine for Rescue which Defenders get that others don't. BTW, it's nice that you tried to give a thorough analysis of how rescue would possibly play out on a mage, but you are kinda missing the point that you yourself are making. Mages are strong enough as it is, they don't really need to be any stronger. If they can't optimally use rescue, oh well. Not like defender can optimally use half its' abilities anyway. Who cares if a mage has 1 ability that is hard to use optimally.
Mages already have abilities which are "hard to use optimally" - see: the entire Fire branch of the summoning tree, which summons things which are either too weak for where they are in the tree or way too expensive to call. Also see anything that isn't Evoker or Prodigy or Savant in the damage side of the tree.
Also, let's consider what Knowledge would buy a Defender - after gaining the experience required for a normal champion to attain level 12, you're actually level 13 and have as many real traits as that level 12 normal champion. This means you have a whole 3 health, 1 accuracy, 1 spell resistance, and 1 spell mastery more than that champion. You might have an extra 1 or 2 damage, if you found one of the rare weapons with a damage per level bonus. At lower levels, you're effectively weaker than the level 12 normal champions, and you don't really gain too much more from continuing to level up - you've probably taken almost everything you wanted out of the Defense side of the trait tree by now, and the Spell Resistance line tends to have too little use for the passive bonuses to really be worthwhile even though some of the active abilities might be nice. Some traits from the General tree might be useful, especially if there's a decent Magic tree to go up, but for the most part a Defender (or Warrior, or Assassin, or Commander) champion is complete by the early teen levels. Warriors and Defenders start strong, but their most useful traits are mostly low in the tree, and you've probably obtained the others by the time your champion gets to the mid-teens, which means there isn't much point in advancing them past that point. Knowledge, however, is only just starting to pay off by the mid-teen levels - just when your Warriors and Defenders don't really need to level up any more (Assassins are a bit of a special case, since they kind of need everything out of all three main branches of their entire tree, unlike any other class in the game).
Let's compare this to what Mages have - by the early teen levels, Mages probably have more or less everything out of the side of their tree that they decided to specialize in, or they have access to the higher levels of their spellbooks, or they can have, say, Evoker II and Blizzard (note that this is an example, and I haven't checked the game to see exactly what you could get - this is purely for comparison purposes). So with Mages there's an actual benefit to continuing to level up at least a little more. And unlike Defenders, Warriors, and Assassins, the primary attack of a Mage (the spells that the Mage can cast) have a defense mechanism that scales with the level of the defending unit - which means that unlike the level bonus to accuracy, the level bonus to Spell Mastery is actually useful (and yes, I acknowledge that there are some cases where the level bonus to accuracy is actually useful - but it's much more of an edge case than those where the level bonus to Spell Mastery is useful for a spell caster).
Also, you can't really compare mages losing a few % with classes that lose their entire best lines (defender/trainer) to other classes (defender --> commander).
I disagree that Trainer was the best trait line for Defenders - three or four levels put into traits that give a 10% experience bonus per trait, which aren't likely to come until you've attained a level where the additional experience isn't likely to gain you any more levels any time soon, and which still isn't sufficient to overcome the penalty for stacking champions? No thanks. And if you're using Trainer to make the regular troops stronger, you've essentially made it so that your troops will outshine your Defender in all but a few cases - troop health grows far faster than Defender health ever grew (even the average three-figure unit gets 6 health per level and at level 1 has 24 health - a base health of 6 and 2 health per level, all multiplied by the figure count, before any other bonuses - and even in FE where the Endurance traits each gave +1 health per level a Defender couldn't have any more than 8 health per level, which is equal to the average four-figure unit's health-per-level bonus), troop armor is probably on par with champion armor, and troop damage is at least on par with champion damage unless you get a really good weapon. Taking Trainer essentially amounted to "I know that the troops are superior to the champion I chose to play with, so I'll get a few levels on them so that they can outshine me (a Defender) at the one thing I'm supposed to shine at - tanking".
Fireball is still 1 and Horrific Wail is still 2. I don't know which spells you are talking about, but these major ones stayed the same, and by that I mean before the -1 cast time. In LH, mages using these spells do tons more damage than the FE mages using these spells and these were arguably the two most powerful in the game in FE. Note also, these spells didn't get a damage nerf at the same time everything got a HP nerf, that widens the gap by itself.
This is incorrect. In FE, the Evoker traits were +25% spell damage per level, though there were only three of them; on top of that, Warlock was +50%, and Path of the Mage granted an additional +50% spell damage. I may be bad at math, but it would seem to me that a +175% bonus to all damage spells clearly outshines the +105% bonus to all damage spells; moreover, FE champions and LH champions don't seem to progress at significantly different rates (if anything, the LH champions seem to progress a bit more slowly). The only real difference between the games, as far as mage champion spell damage goes, is that in LH you can guarantee that you have a certain amount of bonus to your spell damage by a certain level, whereas in FE you only have a probability of having a certain amount of bonus (excepting Warlock, which you either do or do not have from level 1; FE Warlocks or non-Warlock Mages are additionally on par with LH Warlock Mages with Evoker I even if the FE Warlock isn't a Mage and the FE non-Warlock Mage has no Evoker traits, an FE non-Warlock Mage with Evoker I is on par with an LH non-Warlock Mage with Evoker IV or an LH Warlock with Evoker II or III, and an FE Warlock Mage without any Evoker traits is on par with an LH Warlock with Evoker IV and clearly superior to an LH non-Warlock Mage with Evoker IV).
In short, Mages didn't lose "just a few percentage points", they lost about one-third of their damage potential. In exchange, you can now more easily guarantee that they'll have a certain damage bonus by a certain level - but it falls far short of FE's +100% spell damage bonus on a level 4 Warlock-Mage, which is comparable to LH's +105% spell damage bonus on a level 9 or so Warlock-Mage; moreover, FE's level 4 Warlock-Mage can continue to gain additional damage bonuses, while LH's level 9-ish Warlock-Mage has capped their damage bonus for anything but Fire spells (which can get an additional 20%, for a 125% bonus, which the level 4 Warlock-Mage can potentially match at level 5 if you get offered Evoker at that level up - and the FE Warlock-Mage is still better off since they have the 125% bonus to all damage spells, not just Fire damage spells). And Mages in LH certainly are not comparable in their damage potential to Mages in FE unless there was a spell damage buff between FE and LH (and it's only really Defenders who had significant health nerf, so there isn't an argument for relative damage potentials either).