Not too keen on this. Perhaps for #1 contender it makes sense. To fight the champ you have to fight like the champ. But for a normal fight that happens to be a main event, I don't think so, may take the edge off of a Championship fight.
I know what your saying it would take something away from having the championship rounds with the title on the line, so I am also on the fence on this one.
But at the same time it could add something to certain fights, because I know I have seen fights before that have been non title that I would of liked to see go the extra rounds, but having said that its not always the main event that turns out that way.