Like a few others mentioned, the animations would be wonky as fuck.
I think you should be able to fight open weight top to bottom if you want. It's a game and people buy it to play it how they want to.
But I used to do the easy open weight glitch on UFC 3, and if you'd take Cain and whip Demetrious Johnson around, there'd be pretty awful clipping mechanics and also many times where guys wouldn't actually touch with takedowns and stuff because the models and dimensions were so different.
The stats, at least in UFC 3 were also relative to the divisions in a lot of ways. If open weight was a normal thing, flyweights would need like 30 something strength and 90 something speed, while heavyweights would be the opposite. As it is, there is a noticeable variance from small weights to large weights, but it's done within the context of the divisions where everyone's stats are nearly uniform.
But yeah, I think they should do it anyway. Who gives a shit.