This isn't exactly the reason why they need to change how weight classes work, but it is one good reason among a few others.

I would like fighters to weigh in as close as possible before their fight starts, so less than an hour before. The trick is that you'd have to change the athletic commission rules (I assume), and you'd also have to re-negotiate contracts, and created a new system of weight division classifications, and you'd have to set up a system of incentives, punishments, and ways of handling what happens when a fighter does miss weight in such a way that would make it worth-while. The fighters would be more likely to feel better, and have better endurance when they fight, and their listed weights would all be much higher. It would be better for fans and fighters, and in theory that would be better for the promoter in the long run as well, but the promoter definitely takes the biggest risks and costs of trying to implement this sort of change.