You'rd right; I can't speak intelligently on the number of dealers using illegal firearms. It's about not leaving it up to chance though. They may know, to a relatively high degree of confidence, that the drugs are there, but they may know little else. You don't want to raid a home and come up empty handed.

I can't imagine a scenario in which finding a small percentage of the drugs wouldn't hurt the case though. If you can't find it then you can't charge them with possessing it.