The argument for debate inclusion is simple, logical, and accessible to even the most partisan among you; the 15% poll threshold is purely arbitrary, we need broader political engagement, not narrower, and if a third party or independent candidate is able to land on enough state ballots to theoretically have a chance of winning, what's the issue?
Apart from the needs of the two-party duopoly, that is.
Jill Stein Should Be Part of a 4-Way Presidential Debate, by John Nichols (The Nation)
Even those who do not back third-party contenders should recognize that open debates are vital for robust democracy.
... But this is an uncommon year in American politics. Libertarian presidential candidate Gary Johnson has already getting close to the political high ground where a third-party candidate is treated as seriously as the nominee of one of the two major parties. And Stein is climbing as well, having just appeared on CNN for a prime-time special highlighting her candidacy. Stein’s progress is significant because, in order to have a real debate in American politics, it is vital to include voices from across the political spectrum.
A debate featuring Democrat Hillary Clinton, Republican Donald Trump, and Johnson would represent an improvement on what we have seen in recent presidential races.
A debate between Clinton, Trump, Johnson, and Stein would represent an even greater improvement.