[J_Darnley]

Have you ever seen cases 2, 3, or 4 happen?

2 - Why would a site use some sort of middle layer just to ensure that links remain permanent? They could just redirect old URLs to new ones.

3 - I am aware that the owner can change the URL behind a shortened one so if they needed to they could fix "links" to their site. I have never heard of a service which claims to find out where broken redirects should now be pointing.

4 - I think they make attacks more likely. Most people's browsers will automatically follow the redirect and not give them a change to say no if they don't like the look of the URL. Yes, in theory, a user could try to report a dangerous link but I would be surprised if anyone is available to listen at these services.