The main argument against URL shortening services is that users don't know which domain they are being redirected to.
quote taken from this article: http://research.zscaler.com/2010/03/are-url-shorteners-really-dangerous.html (link article author = Zscaler Analysts)
Opposite opinion from poster of this link, LedaOhio5, here: http://www.godlikeproductions.com/forum1/message1915584/pg1
and a huge proportion of shortened links are just a disguise for s p a m