Sure, but the relatively short link to your reply, http://news.ycombinator.com/item?id=2492783, is already 44 characters, so I'm only left with 96 characters on a twitter message.
With bit.ly, the link is much shorter with 21 characters, http://bit.ly/j8IpC8, and I'm left with 119 characters to describe the link in a twitter message.
Oh, and I'll know how many people clicked on my bit.ly link. I've become addicted to that feature.