If the contractor does the job well but can't pass the usual interview, then either the contractor got unlucky or the interview process is broken. If the interview process is broken, then I should fix it for everyone, not make a one-time exception (e.g., realize that my front-end developer doesn't actually need to know how to balance binary trees on the whiteboard or whatever).

That leaves the possibility that the contractor was unlucky, since even the best interview process will generate some false rejects. I believe the extra familiarity that a quasi-internal candidate has with the group's work (and thus the kind of stuff we're likely to ask) should more than offset that, though.