Yes, you only get the bonus when one of your freighters is in range. The value of adding the trade modules depends on your play style and goals. Most of the scoremonsters seem to put little emphasis on trade revenue and some folks never establish trade routes at all. I use trade mainly for diplo help, but once a route is there and I build a starbase for a manufacturing boost, I usually end up filling out the trade slots just to use up local constructors.

I've been playing a long time, and I still sometimes feel like this about them b/c there are some conflicting takes on how they work around these forums. AFAIK, they are mainly about expanding your influence territory and pushing rival civ worlds into flipping to your side. I've never studied what they do to my own worlds carefully, but I have an informal impression that they can help defend a world that's deep in rival territory and vulnerable to a culture flip. (I have a world flagged as at-risk, I build an inf starbase nearby, and after it has a few good modules, the at-risk flag on my colony goes away.)

As a note, trade is +x% (in this case +10%), which is one reason why so many of us consider it so useless, as income from trade is quite minimal to begin with.

It can be a powerful defense, but it works by generating its own influence. Think of it as building a handful of temporary influence-producing planets to add weight to the one at risk.

If ISBs only enhanced the influence of your planets, this wouldn't work very well. You'd get a large % bonus, but that bonus might only apply to a tiny influence base if the planet you're defending had little population. ISBs work independant of their surroundings, which is nice when the surroundings are not too friendly influence-wise. This also explains why you can open a patch of friendly territory with ISBs even when your nearest planet is two sectors over.

I didn't play GC1, but the number of influence sources must have been fairly low. In GC2, planets might have 100+ sources of influence within a couple sectors. Still, it might be nice. It certainly would have made putting all of this together a lot easier for me.