Nominees from parties like the Green and Libertarian Parties deserve to debate. For instance, the Libertarian nominee now routinely polls relatively highly (when included in polls). The rule now is that national polling must show a candidate at 15%, yet how can they if they aren't listed in the poll?
That's unfair. When third-party nominees are usually not included in national polls, the standard should be changed. The White House should seek to see this threshold lowered for 2016 and beyond and ask that other reasonable standards be put in place (e.g. being on the ballot in all 50 states or something).