One downside could be the extra tracking and analytics URL shortening services provide, and people could see this as an invasion of their privacy. Personal belief is that there is so much tracking in the form of web analytics that goes on on the internet anyway that URL shortening is hardly making a difference. It has also provided some businesses like bit.ly with a way of generating revenue and creating jobs and for bloggers and people like me who do like to share what I find interesting on Twitter or Facebook with a way of seeing if anyone is actually reading what I have shared