The problem with cool URLs
Back around 1999, Tim Berners-Lee wrote Cool URIs don't change, which is about how your URLs shouldn't change (and he gave advice on how to manage it). People have been nodding sagely every since (I hope, since it's a good idea). But there's a problem.
The problem with cool URLs not changing is that it means that your URLs are forever. This means that you either have to get the URL right before you publish it for the first time (leading to taxonomy issues among other things), or you have to keep supporting the old crufty ugly URLs forever (even if they just give HTTP redirections to the new URLs).
Most people aren't going to get their URLs right the first time around, because structuring information is not a small and simple matter (despite how it looks; ask a librarian about it sometime).
Supporting old URLs is a deadweight on your web environment; it's a kind of clutter. Clutter makes things harder to maintain and to keep track of. (Plus you have to actively avoid namespace collisions between new URL schemes and old URLs, which may constrain what sort of new schemes you can use.)
Ironically, you can argue that the best long term approach is more or less meaningless URLs, plus searching and navigation to let people find things. When a URL doesn't mean anything to start with, there's no temptation to change it because you've realized that the meaning is wrong.