There's something to be said for simply polling high enough to get into the debates. Debates where a candidate could, for example, offer an alternative view on drug policy.
If a third party gets 15%, they get a third podium on that stage, which I think would be worthwhile and healthy for American democracy especially in the face of the two most hated candidates in history.
I don't particularly care if it's Johnson or Stein but I'd like it to be someone. Ideally we'd have both. Johnson is pretty close to breaking 15% in a lot of polls.
Of course this line of reasoning simply suggests that pre-debates, if anyone asks you should /claim/ to be voting Johnson (or Stein), not anything about where you should actually vote.