I agree with the sentiment, BUT, you are giving fighters way too much credit.
This, fighters are going to push the limits of their own health in order to gain any advantage possible.
It's not possible to put the weight back on in such a short space of time. If they try, the fighter cutting weight will be at a *massive* disadvantage compared to a dude not cutting. In other words, fighters wont cut. Sure, they might find other ways to gain an advantage, but dropping 20 pounds and putting it back on again wont be one of them.
IF they we're to do pre fight weigh ins, what we'll see initially is a bunch of fights called off on fight day because a fighter has attempted to cut weight and blown it. This isnt a good thing. But with time, the fighters will realize that trying to fight at a weight class well below their natural weight is futile and will stop doing it.
Besides, if most fighters move up in weight then nobody will gain and advantage, right? Only a select few ( Edgar, Machida etc ) currently fight at their natural weights. These are the guys that will have a big advantage initially. Completely fair in my book.