They really should give us that flexibility. Playing UFC 3 right now, I can't make Chael Sonnen fight as a light heavyweight, or Machida fight as a middleweight, and several others who have gone up or down a weight class.
But for some reason, whenever someone brings up weight classes, the conversation always turns to "We can't have Mighty Mouse fighting Roy Nelson!" Even the developer used a similar example in his Q&A on reddit. But that's not the kind of match I give a crap about--I just want to be able to recreate fights that have happened or will happen in the real UFC.