Sorry I wasn't clear. YOU lose buoyancy when you let the air out of your BCD. That loss MIGHT be compensated (or even overcompensated, as you point out) for by the DSMB....if you hold onto it. If you don't or the string breaks, or the DSMB pops/leaks, you will sink. So you need to be prepared to put air back into your BCD, because your buoyancy is now split into two places and one of those places is at risk.
I'm not saying it can't happen, but how many people have had there string break solely due to tension during ascent of the DSMB??