Maybe you want to come up fast?
Maybe you want to practice ditch and don of all scuba gear on the bottom (Ive done this dozens of times too).
Maybe you have a total BC failure and decide that you want to remove some ditchable lead, if you have little or no wetsuit on, then you will not experience any change in bouyancy on ascent (I've had a bc totally fail before).
I find the comment that there is no reason to ditch a belt underwater to be incredibily short sighted.
The "fastest" ascent that PADI teaches is the CESA, and for that you would definitely NOT want to ditch weights at depth, unless you're so overweighted that you can't swim your rig to the surface. Why would you "want" an uncontrolled ascent?
Practicing ditch and don at the bottom doesn't have anything to do with dropping weights at depth, does it? So, you practice removing your weight belt, fine, but you're hanging on to it or something else specifically to prevent what would happen if you dumped your weights at depth.
If you have no wetsuit on, you should only have enough weight to compensate for the inherent buoyancy of your rig (including empty tank) and yourself. Again, if it's not very easy to swim to the surface, you're overweighted. Total BC failure simply means you have no positive buoyancy; it doesn't add to your weighting.
The theory that you "should" have the option to ditch weights at depth in some bizarre emergency in which you are overweighted, buddy-less, and out of air is not by itself a problem, I suppose. But, in practice, you have to balance the possibility of accidentally losing weights at depth and the associated danger against any theoretical "safety" that's gained by having ditchable weights at depth.
