The Libertarian party is much bigger than the Green party, and there are many people who sympathize with libertarians on a lot of issues even if they don't call themselves libertarian.
Johnson/Weld are real candidates -- both two-term governors from moderate states! I'm surprised they don't have more support considering how bad the major candidates are.
Regarding policies, Greens and Libertarians are also 99% in alignment on this particular issue (war on drugs), as well as some others. I know people have their preferences, just pointing out that there are multiple similarly-sized third parties of yet complete opposite ideological natures that would still be optionable for those wanting to vote on this kind of thing. It's even easier to not support this kind of behavior by the DEA.
http://www.economist.com/blogs/graphicdetail/2016/09/daily-c...
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.
Not to mention the CPD would almost certainly change the debate qualifiers immediately to still keep Johnson out, same as the DNC did with Lawrence Lessig.