When you have a winner-take-all system of voting, third parties tend to siphon support from those who are ideologically closest to them. That's not a good way of choosing representatives of the public.
Libertarians, greens, tea partiers and progressives should join hands and support Instant Runoff Voting, Approval Voting or any number of more rational election schemes. Until they do, it's appropriate to dismiss minor parties as spoilers. Absent such reforms, I support the decision of ABC. Otherwise, I would say that there should be one debate for those polling above 2% and another debate for those polling above 20%.