I don't think this really benefits users all that much from a privacy perspective; it really just consolidates tweet link clicks in server logs.
The information that would normally go in the referer header would presumably be available via future t.co APIs (although there is some privacy protection there now).
Posters can still use bit.ly links (just not by default); they would actually have a t.co link to a bit.ly link to their actual link. t.co links may offer similar statistics in the future. Twitter is also a private company that can track a large portion of peoples' browsing history.
Very true, though I'm still not sure what I think of the balance between privacy and business-friendliness that is best - just like most countries, I suppose...
The proportion of sites with Facebook/Twitter/insert-hot-social-site buttons/widgets on nowadays means that a great deal of people's browsing gets tracked anyway, unless they take precautions against that. Pretty much every blog, startup, news site, etc etc.
It masks the referer header, which protects my privacy without breaking sites that rely on the referer header.
Secondly, and much more importantly, it gets rid of privacy-destroying URL shorteners like bit.ly that give the posters statistics on their tweets.
It might be a tiny annoyance to some developers, but the privacy gains are fantastic.