Why not? Even after unbuckling the harness belt I have to struggle to get rid of my BP/W. There is absolutely no way it will come loose accidentally. Besides, you wouldn't want to drop the weights before reaching the surface under any circumstances.
I haven't had much experience in a Bp/W. I'm currently saving up for one and just generally asking questions about techniques and anything else that could come up when I do get one.
So as of right now that's my opinion. I'm sure it'll change when I get more in water experience with one. It just seems to me that unhooking the crotch strap would make the plate less stable if the wing were inflated. It could shift off the center of the diver's back when you're bringing them from depth. It just seems to me that the diver and their Bp/W are no longer a single unit once you take the waist and crotch straps off.