Not everyone dives with integrated ballast. Many, myself included dive with a rubber weight belt with hard lead as it makes it easier to remove your rig at depth and not float away from the BC. Also it makes the rig lighter for the guys who pull rigs over the gunwale. Nevertheless, if you accidentally put the crotch strap over your weight belt you've captured the weight belt and would need to unbuckle the crotch strap first. In an emergency on the surface a diver may not have the mental wherewithal to do that. Conversely, with the belt over your crotch strap you may have to remove the weight belt first to take off your rig.
For all the aforementioned reasons I personally ditched the crotch strap on my rig and have not noticed any negative effects.