I think there are some pros and cons to each.  I know a ton of people that just stay logged into ventrilo (a common voice server) all day.   I’m not that enamored with the voice chat, but it is definitely useful in World of Warcraft.  Certainly for action and socialization, voice chat is pretty cool.  Text chat is kind of a nice way to retain anonymity and casualness.  Anyway, this article from wired.com is pretty interesting:

Voice Chat Can Really Kill the Mood on WoW