Yesterday Keith Dsouza wrote in Weblog Tools Collection about the problem with short url in comments which are used by spammers to try to get their comments approved.
The problem with short urls is that in order to know where the link points you have to click on it with the risk of being sent to a malware site or something else. Or you can use another web service to expand the URL and check the destination but you have to open another browser tab, copy the URL from the comment and paste it into the other tab.