1. Locking them out of the debates is a big reason WHY they are under 15% support. If they were in allowed in the debates those numbers would likely be considerably higher.
More often than not the opposite is true. The support for the third party candidates comes from the disgust that the voters have for the two main party candidates, such as this year. But when the third party candidates positions are examined closely it becomes obvious why they are a minority party, because they don't have a clue how government works or how weak their policy positions are.
The Libertarian Johnson believes that we should take the long term view of global warming, in some billion years the sun will expand to engulf the earth. And the Libertarians are the foremost believers in the faith of the never before seen self-regulating free market. Their assertion that the economy would operate better with minimal government involvement goes against all of history, highlighted most recently by the Great Financial Crisis caused by the government's ideological induced failure to regulate the banks.
Jill Stein's signature policy proposal is as president she will order the Fed to bailout the people having to pay back their combined over 1.5 trillion dollar student loan debt. The president can't order the Fed to do that. Besides, the student loan debt problem requires a better long term solution than a single massive bailout. Private debt is a massive problem for the economy because it destabilizes the economy.
2. Even if their chances are minimal, their voices can still be extremely valuable, because they can elicit responses form the main contenders. There are many issues that go completely unmentioned in these debates. But if one of the minority parties had one of these issues as their main platform point, then the main parties would be forced to take a position on them.
Do you have an example of an issue raised by a third party that isn't addressed by at least one of the major parties?
Granted, Trump and the Republican party don't address a number of issues, for example, Global Warming, income inequality and the student loan problem, among others, not too surprising since they created a lot of the problems that need to be corrected. I suppose that their party default position of "no" is suppose to cover the issues that they don't address.
How about allowing the minority parties into the debate but allocating questions/time to them based on their support? If you are polling very low you only get to show up for a few minutes, basically as a guest speaker in the debate for your primary issue, and then you're not there for the next segment?
A proposal that no one will agree to.