I think the crux of it is that voice chat seemed the rule rather than the exception at one time.
As a member of a guild that spends most evenings questing together, it can be jarring to do a morning quest or two with a random group, none of whom use voice. Unless I don't feel like using voice, I will use the mic a couple of times just to "break the ice". This usually seems to pull one or two people out into voice chat, although with less frequency lately.
Nobody has to use a mic, or even type for that matter. Some folks are quiet throughout, but respond to instruction and exhibit good teamwork. The fact of the matter is that communication in the game can be poor. Voice chat isn't ideal, but typing in the midst of an important fight is tough sledding. When things get hectic, people are probably listening to the guy on voice instead of anyone typing. The conundrum becomes: how confident do you feel in letting someone else do the driving?
My personal feeling: it's kinda nice to relax chatting with a group of folks.