i mean, why you use shorten URL http://bit.ly/dIROp7 instead of http://itunes.apple.com/sg/app/stack-on-me-expert/id417621861?mt=8 Sorry for speak like Alien
I don't really trust short urls myself but sometimes I've had fun with http://www.shadyurl.com linking something innocent enough but with links that freak people out. Not on forums though just for my own amusement. In answer to the original question though I'd think the char limit myself.
3 big reasons: 1. Long URL's eat up the character limits of signatures fast. 2. URL shorteners allow click tracking to see how many people are clicking your links in your sigs. 3. Evading URL filtering that we have for some sites that insist on incessantly spamming or otherwise breaking forum rules.