Currently it's possible to link to a profile in two ways:
site/profile/username
site/profile/id
Having the second method is potentially problematic. If a user for example changes their permalink to a number, suddenly there is a serious possibility of their user permalink being the same as a user id. View this profile:
http://www.test.alpineascent.com/63
Using my login details, use the stacked account to change the permalink to a non number. Suddenly you will see that link to to a different profile.

I admit that because the permalink has precedence, the issue is minor. However I could see this creating potential conflicts which it's always good to create less issues for you guys to deal with.
The question arises, is the structure site/profile/id required? I'm also trying to be conscience of Google potentially treating it as a double record which could lower SEO.

On the flip side I suppose it might only be viewable when you actually query it via the id in the url. Hence making it possible that the separate id page might be hidden from search engines?