Last I heard, there was a larger percentage of the community that was neutural/against the idea of a built in voice chat than pro built in voice chat. At least that's the statistics that Lyte threw around.
I personally don't want to hear the demeaning language that this community is likely to spew out over an in game voice chat. I don't start my games assuming my teammates will be jerks by any means, and that tendency leads me to normally not mute people in the hope they'll turn around or say something helpful in chat. I'm fairly certain i'd feel obligated to treat voice chat the same way, and would be a bit reluctant to mute people.
Also, voice chat with strangers in League doesn't seem as helpful as you'd think. I've played with strangers for some tournaments, and we've used Teamspeak there, and you spend the whole time trying to match voice with name. It's much easier to be on voice chat with friends, as you already know their voice and what champion they are, so when they say "I" or "me" you know who that is.
TL:DR; Don't want built in voice chat.