when you consider how many UFC fighters seem to enjoy taking a hiatus to make movies, it only makes sense to have WWE and UFC swap talent. working in WWE and making a movie is the same thing: cashing in on notoriety in the entertainment business.
super long term, it seems like WWE might get out of the ppv business entirely, which means they will no longer be a "competitor" of the UFC in that sense. its a good marriage.
and making "one night super fights" makes perfect sense for the UFC. not every fight can have the same stupid narrative of "im working my way to the title." some fights just need to be for fun, like the old UFC super fights or the pride model.