"We have no idea how you're playing the Minecraft," Notch wrote on his blog today. "Right now, the only way we can figure out roughly what people are doing with the game is to track logins. Once you're logged in, we have no idea what happens."
I do think its great that he asked before doing it, but I am sorry its not information he needs.
Cause if you want to figure out roughly what people are doing with the game... Look on your forums, and all the various other websites regarding what people are doing with the game be it in game projects, servers, modding, etc. So Im sorry, No Notch, you DO have other ways of doing it, so you have no justification for trying to be intrusive.
Seriously no good can really come of this as no good ever comes out of people wanting to take data. Besides that, Notch has repeatedly illustrated hes just going to do whatever he wants to do with the game anyway So realistically having that data serves no purpose.
EDIT:
The fact that so many people from this generation are so apathetic to freely giving up their information is beyond distressing and explains a LOT.