He is American born, meets the age requirement and represents a legitimate, recognized political party. It is a disservice to the American people for the presenters of the presidential debates not to host a debate that includes the candidate of at least the next two most popular political parties. If the major networks do not want to broadcast a debate including secondary candidates then a separate debate should be aired so the American people can hear the views of these candidates.
Gary Johnson had views significantly different from the mainstream political perspectives that Obama and Romney had, and his voice being heard in the debates would've given people a much clearer idea of what their choices really were. As long as we remain slavishly devoted to our two-party system, the true elites can use a strategy of "divide and conquer" to keep partisans on both sides fighting over petty issues, distracting the people from the fact that two-party rule is practically one-party rule.
I think that anyone should be allowed to participate in the Presidential debates. It cannot hurt to have more opinions on how we should run our country. With more opinions bouncing around then maybe we can actually get some stuff done, and get our country out of debt, and back to the number one spot.
While he doesn't pull anywhere near the amount of support that the other two candidates do, he does have the third largest base behind him, and it seems only fair to at least allow him a position. He's on the ballot in most states, has... some support, and in general is a serious enough candidate that he should be granted a degree of participation.
While he may not be a major candidate, Gary Johnson is a slightly influential candidate, and should have a part in the 2012 presidential debates. Johnson and his party both have focuses on issues and concerns that should at least be voiced and brought up with the other two major candidates.