Most people live in uncontested states (for the presidential election). Voting for a major party only makes sense if you actually like the candidate, or you live in a swing state.
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 size, the official memberships from 2014 were ~411,000 (Libs) vs ~248,000 (Greens). You could look at that as either the Libertarians being 66% larger, or them being practically the same as far as order of magnitude and portion of the total electorate. Johnson/Weld are more experienced candidates than Stein, but on the other hand, Ron and Rand Paul never came as close to winning a nomination as Sanders did, either (him being about 98% in line with Stein's positions - I remember his local campaign staff even defecting to her around the time of the DNC endorsement).
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.
Party membership numbers are wholly irrelevant -- the number that matters is electoral support, of which Johnson/Weld has 4x that of Stein and drawn in substantial numbers from both major parties.
If what matters to you is actual electoral support, there's no reason to be voting third party anyway. Both Stein and Johnson got less than 1% of the vote in 2012, and even combined only about 1.2%. The most successful third-party/independent Presidential candidate in decades was Ross Perot in 1992, and even winning just under 19% of the popular vote he failed to win any of the electoral college. It's a self-serving double standard to tell people to pass up the major party candidates but then vote based on electoral support for the third parties, ignoring how well their ideological and policy positions actually align with your own.
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.
It does matter that it's Johnson over Stein. Johnson and his running mate were both two-term governors. If they make it to the debate stage, they are real candidates and nobody can dismiss them.
You (along with the immediate parent) actually do have a good point here. If any pollsters ask, I'd probably say I support Johnson for this reason, though overall I'd personally prefer Green policies over Libertarian ones. There's something to be said for Greens/Libs banding together just to help wedge third parties into the process at all. This kind of strategizing is still pretty unfortunate though, and only makes sense for the polling/debate process, not necessarily the electoral one.
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.
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.