Short URLs are valuable in instant messaging or other text length limited places. They are also a valuable method for marketers to track which marketing tool produced the best results. I actually didn’t know that second bit until I checked out http://bit.ly/pro/.
Short URLs mask the real URL from the user. When a bot or a stolen account inserts a link into an email to a list, or a comment on a web page, or anywhere else, users may follow the link to a dangerous web page or an unexpected porn site.