Should not be for the belt. Nate hasn't earned it.
I'm biased, so of course it should be for the belt.
That aside, the UFC has a precedent in this scenario. I understand why people say Nate shouldn't fight for the title, however Nate and Conor are becoming the UFC version of Batman and the Joker. Nate vs Conor for the belt, Ferguson v Lee for the interim, winners fight..... Just like how they did Big Nog v Mir for the interim, and when Randy came back, had him fight Brock for the title, then unified them.
Yes it devalues their titles, but it makes them more money, and right now they're all about trying to make as much cash as possible.
All that said, WAR DIAZ!!!