I just completed Assassin's Creed 2 and so I'm looking at tying up loose ends (ie - finishing trophies/achievements). However, I'm surprised to find that there are no trophies for getting 100% of the treasure chests, and yet the game actually tracks how many you have gotten (under your DNA). Is there a good reason for taking the time to get them all?
I suppose it depends what your motivation for completionism is. I collected them all just "because they're there", but in terms of tangible rewards, there aren't really any. They all just contain money, which you probably no longer need more of, and as you said, there's no achievement for doing it.
So if you just want to do all the achievements, no, there isn't a real purpose to getting them. But they are a collectible that the game keeps track of, so if you want to fully "complete" the game just for personal satisfaction, I'd say that they're a component of that.