Enpi's ideas are pretty much dead on what I would be looking for. I don't think he means the battles just happen in the background. Maybe that can be the case if freighters run into raiders unprotected where you randomly lose some and you just get a report. But the idea would be if escorts are present it sets up a battle the player has full control over just like any other battle in the game between escorts protecting freighters versus raiders. Heck, this could even lead to special "Q ship" hulls designed to look like freighters but armed like warships to act as escorts for real freighters and surprise raiders.
As far as fluff goes there are plenty of ways to make it work from having raiders require gravity projector modules to drag freighters out of hyperspace to having battles take place at random colonies/planets with freighters present at the edge of the map to protect/defend some distance away from the actual colony and any defenses there might be there.
As far as gameplay value goes I, and apparently others, seem to think it adds plenty. Especially for weaker powers to inhibit the economic strength of stronger ones or for races/players who just like limited warfare in a run up to full on battles. If I've got a few spare destroyers sitting around I'd love to be able to turn them into raiders to weaken an enemy. And if I see an enemy investing heavily in raiding me I'll either invest in a lot of escorts or attack him directly to remove his industrial base for building more raiders.
This will be the last I say about this topic. I feel the points have been laid out for-and-against such mechanics in the game. We'll see what the future holds.