Chat messages are simple and more controlled, not to mention it's just overall better. And anywho, you'll have to mute a bunch of people at one point. Consider these:
-If someone's just blaring music in the background, do you mute 'em? We all know that some people like listening to music while they play.
-Will we mute people when their family is being loud in the background because they don't have computers in their bedrooms?
-How can you censor the chat? You'll have to ban or mute the guy spewing a gratuitous excess of cusses
-How are we to drama people? Will we have to record them, and have Badspot add a feature to the forums whereby you can add audio to a post? Do we have a chat recording ability so we are able to do so? We might not be able to hear them over other people.
-How are we to censor it for the soccer-moms who are afraid of their little darling child hearing someone say something inappropriate?
-Will there be a mic sent to you when you buy Blockland, and will those who have one already need to drive to Badspot's house to get them? Will there be a Blockland approved mic, or will everyone buy their own?
-What happens when there are thirty or more players, all trying to get heard by screaming? Even fifteen people is chaos.
-How are we to tell which voice belongs to who in a server with thirty players? You can't remember what everyone sounds like.
-Suppose there's an argument. Do you ban two people just debating worthless bullstuff, or do you mute them?
-Suppose someone has a cold and is coughing/sneezing constantly. You won't be able to hear anyone over them, so you'll have to mute them, won't you?
-Suppose the server lags. Instead of idiots typing "LLLLLAAAAAAAAAAAAGGGGGGGGGGG!!!!!!!" You'll hear idiots screaming it.
-Consider the problem of team chat for deathmatches and voice commands and chat-related events and mods.
-Consider the problems for IRC.
-Consider the fact that there's both music and miscellaneous sounds on Blockland. If you're next to a brick playing loud music, will you have to shout to be heard over it? What about in a DM with thirty players, with bullet sounds coming from every direction?
-Consider the fact that adding a mute feature won't loving fix everything.
Tell me what voicechat would be like on this server:

Or rather, imagine
any of Kalphiter's hosted servers, that host up to ninety nine players.
The higher the playercount, the higher the level of insanity. Over fifty prepubescent forgets all screaming conveniently into your ear. Fantastic. By no means is voice chat as perfect as you people make it out to be. Teamspeak is fine, but that's where it should end.