In some situations, like a failed bladder in a BC, in may be preferable to find a way to get more positive quickly.
Dumpable weight doesn't have to be on a weight belt. Most BCs have a way to carry weight that can be dumped.
That being said, I still use a weight belt most of the time. I also frequently carry some weight on my BC. Weight pockets are available that allow you to add trim weights to any 2" web. My current deep water rig is 6# on the BC & the rest on my belt.
I prefer to carry my weight in more than one place. If I do need to dump weight, I will probably prefer not to dump all of it at once, especially if I am wearing a thick suit. Loosing 20# all at once at 100', can present significant challenges when trying not to pop up to the surface uncontrollably at a deco stop or safety stop. In fact, making the stops may become impossible if you have dumped a big enough chunk of weight.
Dumping weight in a panic because you want to pop to the surface quickly is a very dangerous move. Accidentally loosing a very heavy weight belt at depth can be a dangerous situation. All risks must be considered.
