It should be possible to easily search issues if you know the name of a user but not their user id. This is especially useful since the user picker can be very slow sometimes. The implementation should have the searcher index the full name with stemming (I think this will allow minor mis-spellings?) and the searcher itself will simply add another option on the select list. This will of course suffer from the same problems as groups if the user's "Full Name" changes, in that the issues will have been indexed on the old full name. This will require a reindex.