They (the cops) need something to barter with. You're much more likely to get useful information from someone facing 10+ years than someone facing 1 year (6 months with good behavior) or probation. They want to find everything.
In most of the US, and I suspect for most of the population, there is no such thing as registering guns (that would be akin to registering laser printers, especially those high capacity ones).
On the other hand it's illegal for a drug user to possess a firearm, 18 USC § 922(g): http://www.law.cornell.edu/uscode/text/18/922
(it's just that the average consumer doesn't know about it)
To the extent there's any tracking of printers, it would at most tell you which store it was sold from, there isn't any required serial number recording done at checkout (well, as of when I last bought a color printer in 2006).
What you're referring to would be akin to comparing a bullet found at the scene of a crime with one fired from a seized weapon, albeit easier; a comparison system. Much less of a privacy concern, without any required "registration" involved.
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?
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.