Let's face it....fighters are going to try and get whatever advantage they can. Even if weigh-ins become "same day", fighters are still going to cut, and then try to rehydrate.
Why not make weigh-ins 2 days prior to the fight? This gives a fighter much more time to rehydrate between the weigh-ins and actual fight. This would probably cut down on poor performances, and maybe even allow fighters to be a little healthier entering the cage/ring.