Well, I think in the case of Wildsiderz, it was less basing off likenesses (for the main cast, the science teacher is  straight-up recognizable character actor) and more of merging together.  So Jess might be Michelle Trachtengerg, but she had aspects of other people fused in as well.  Which makes sense.  James Marsden, however, dead look-alike.

The issue with doing likenesses, at least from MY experience, isn't just the obvious rights issues (which, not being published, i haven't had to deal with) but the fact that you start having TOO many faces you want to do.  Everybody becomes a character.  So mixing, matching, and mashing-up just makes sense.