Not all link shorteners are evil and are destroying the web. Here are some scenarios in which I like link shorteners: 1) Removal of the referrer (the anonymising redirect) 2) Redirects within a site when content moves, but the redirect service offers a permalink shortened URL. As only they can generate the URL you can trust that the destination is as safe as the source (the intra-site trusted redirect with vanity URL…
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.