It is something I will add, although you might be surprised at how frequently you don't want it to work that way =)
For example, if B moves then you can do things like start dragging off in space, towards the model, to create dents, etc. Also there are some effects that are extremely hard to get if B is not dragged too. Basically if B isn't dragged you will get a lot more mesh self-intersections, etc.
Anyway for some of the other volume brushes, I want it to behave that way, so drag brush will get it too. Although in general this kind of thing is better handled by surface brushes, which I will be adding too.