Just think of Halo and it's voice chat with kids. Do you really want voice chat? And it's tough enough that the servers have to deal with lots of people from around the world. Adding the voice chat would require an upgrade of the servers and more money. Perhaps if they are successful enough and enough people want it.
You can add anyone to Plus+ so you're chatting with strangers who could possibly be a kid or just plain annoying. I'm only looking at the downside of this I know but this world is full of idiots. I hope I didn't add anyone TA that could bug the crap out of me once eliminate comes out. I'm not saying voice chat would be completely terrible but I'm just content enough with text chat.
Plus not every has a mic or an iphone. iTouch users would have to buy one of those itunes earphones that has an built-in mic. Not many people have that either.