You'll find that I'm stubborn about the strangest things, one of which is voice chat. I try to avoid lengthy phone conversations when I can, and only use Vent when running instances (in WoW) with guild members. I've found that voice chat with random people is easily one of the most obnoxious things in the world. Which is why I'm not upset in the least to hear that it won't be included in Mario Kart Wii.
I'll bet that there are plenty of people that will be disappointed by this news, and I can understand. I suppose it would be fun to trash-talk my friends during a race without actually having to see them, but that would likely be the extent of it.
If you're worried about the lack of communication in the game, Nintendo will be providing you with a solution between matches. Don't get too excited though, you'll only be able to select from a list of pre-determined phrases between games.