Um... why? Sorry, but I don't have a problem with the length of the typical site URLs. The forum-post software shortens them for display purposes, and if I'm posting links elsewhere and need them to be shorter I can use TinyURL or some other URL-scrunching site.
If you have a particular reason for wanting shorter URLs, do explain - there may be workarounds you haven't thought of yet.
Because so practiced and is good for when you are printing on paper.
Not sure why you need to put your full profile URL on paper... I never have. The books I release are labeled with their BCIDs and the main site URL, and anyone who enters that info can see my profile by clicking on my screen name - I don't have to write it anywhere. If I want to share my BC profile online it's an easy cut-and-paste, but I don't think I've ever needed to write it down!
Aside from whether or not this feature would be an advantage to very many BookCrossers, the change - simple though it sounds - would require a fair amount of work on the site itself, as there are so many features that make use of the existing URL.
Again, as a workaround, you could use TinyURL or some other link-compression site to make your URL shorter; sure, those URLs look a bit cryptic, but you'd save even more than the 12 characters of "mybookshelf/"!