That's correct. I think the assumed use-case is one where you (A) have a more sane number of IM contacts, or (B) have most of them merged. As mentioned, you can type to filter from the contacts app - or just start typing from the home screen. And yes, if you delete (or disable) the AIM account, at most it should remove the AIM-specific info. However, I think that if the contact was pulled from the AIM account, and not merged/modified (e.g. only an AIM contact), then it might remove those.