Now you're onto something! Your first point is true. If the suspect manages to dispose of even some of the substance, that could hurt the case. Or it may not. There's lots of factors, I was just giving the simplest example I could think of (trying to flush a sufficiently large amount of cocaine).

To elaborate on your own example though, pretty sure that even if a firearm is legally registered you can still get in trouble for having it around, especially loaded, in the context of selling illicit substances.

That is almost beside the point, though. How many dealers do you think are using registered firearms to protect their trade? Of that group, how many are registered to the dealer using it and not some random family-member or whatever?