Look, I was probably a bit harsh on the guy in my last post, but I don't think it would be in his best interests to be in the debate. Iirc he's currently polling at around 10% which is incredibly good for a 3rd party candidate. While there may be some who know of and support his policies, I think a majority of his voters are disenfranchised conservatives, who find the concept of Trump awful, but can't bring themselves to vote for Clinton. Even if the Aleppo mistake just legitimately not understanding the question, this highlights part of the reason why it wouldn't be good for him to be on a stage with Clinton and Trump. He isn't anywhere near as well prepared as either of them, and while he may be just as competent (probably more than Trump), he will look unprepared and uninformed.
Johnson may have been a good governor, but there's a huge difference between that and being President. The President can't just say "oh, foreign policy isn't really in my parties platform, I'm going to ignore it." America's decisions make a huge impact on the rest of the world, and while obviously focusing on America is an acceptable thing for the American President to do, he can't be as focused on it as he seems to be. Your status as a global superpower comes with a responsibility. Coming from an Australian, given that we pretty much follow you into every conflict you're involved in, I find the concept of both Trump and Johnson disturbing, as they just aren't very well informed. While there are checks and measures, and the President can't make these decisions on his own, the kind of people both of them would appoint Secretary of State, wouldn't be good for foreign relations at all.
While I'm very clearly not a libertarian, I don't actually mind Johnson as a person. He's significantly more modest than either of the main two candidates, and he seems relatively charismatic, but while these are both admirable qualities, they aren't really that relevant to becoming President. People like him have a place within your political system, but if he chooses to be so focused on internal affairs, he shouldn't be trying to be President, as they have to care about both America and the rest of the world.