It seems a bit gross when usernames and normal pages are mashed into the same namespace, e.g. https://twitter.com/search I guess Github does the same: https://github.com/pulls Reddit has the nice /u/... thing, but I suppose that is a bit awkward when saying URLs out loud.
One of the reasons why we created this database of huge disallowed usernames: https://github.com/dsignr/disallowed-usernames