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.