Or, I can just add Veteran to the SelectedAbilityBonusOptions. Works great, happens immediately ingame, and I don't need to pop a potion or whatever to see the effect.
Really, though, the +10 XP thing makes a small difference in the early game, but once you've hit the higher levels, when XP to the next level goes up quite a bit, yeah that's not that big a deal. At that point, the other abilities look much more attractive.
So yeah, I use Veteran all the time (add the tag to the Sov code) for playtesting, but I pretty much never pick it in a 'normal' game. Same for units - there are other abilities that are useful into the mid-late game. Sure, the increased HP from L2 is nice, but again a little patience (a few turns) will get you those anyways. Bonuses to initiative, etc, however, are harder to arrange.