I realize that a QR buckle on a crotch strap is a potential thing to fail. But would a QR on the crotch strap make it easier to ditch the weight belt? It seems like it would.
I think horizontally you'd find it easier to release your harness buckle and belt to remove your weight belt, freeing your crotch strap along the way, so there wouldn't be a lot of benefit to the QR on the crotch strap. If you're vertical in the water and your weight belt rides low enough on your hips and won't snag on the rig then maybe you could release the weight belt and crotch QR and not unbuckle the harness. But then you're having to remember two procedures to do the same thing; wouldn't it be easier to just get good at one?
I go for the whole KISS philosophy so I wouldn't feel the need for one personally. On the other hand, aside from possibly being uncomfortable where it's positioned, or making it easy to lose the loop end, I don't see a big deal having a QR on the crotch strap as its not a critical item you'd need to abort your dive for if it failed.