I find that for side missions, I am often not listening/hitting space and waiting to see if any of my options allow me to either slap or flirt the mission giver.
The problem is that all the quests are the same:
1.Go to X
2. kill Y of Z
3. Click 5 blue crates
4. Click teh blue computer
5. Talk to W
6. Oh and while you are there kill 50 of X, 5 of Y and click on 4 of Z (Bonus).
So no amount of VA is going to change that, maybe for the first 20 or so levels, but after that the mask slips and it's a pretty thin veneer of story painted over very one dimensional quests, so if the things that people are saying don't matter (because whatever they say the result for you is the same, see 1-6 above), why bother listening?
This is compounded by nonsensical alien dialogue from about lvl 30 upwards, where they are just looping the same 4/5 'alien' phrases over and over, and the fact that the majority of your character responses are repeated over and over and over.
The class quests are a whole different ball game, as they advance your personal story and you get a genuine feeling of progression, and your responses are a lot less generic.