I take you have never experienced the utter joy that is Xbox Live. Granted, FFXIV isn't nearly as hype competitive as say, Call of Duty, but I don't doubt for a minute a built in voice feature would lead to... shall we say, less than polite responses.

"Why the fuck are you using aoes?"
"Use fucking cleric. We're not here to carry your dumbass"
"WTF? Are you retarded?!"

I could go on.

Typing means people have to stop doing things, which most won't bother with. Voice? Now they can rage freely. And I ensure you, people will.