Well, I would agree with you on the very rare, valuable, or interesting provenance items ("let's rip a few .45 ACP in that U.S. Army trials Luger to see if it works!").
However, on some items with delicate lock work (Colt Lightnings and Thunderers come to mind) functionality does indeed improve value. I like early S&W DA's, and their primary value is as shooters.
Functionality and the ability to safely fire a round are two entirely different things. A Damascus barreled shotgun may be perfectly functional, but completely unsafe to fire. . . because the method of manufacturing the barrel allowed corrosion between the wire wrapping pattern that was hammer welded. . . and over the years since it was manufactured, just by age, it has weakened, no matter how good it looks. Internally, the physically welded joined wires are no longer strong enough to take the pressures. As i told my customers Damasus and other barrels made by similar hammer welded means would shoot from now until Doomsday. . . yours. But no one knows when that doomsday would come as the metal lets go due to corrosion and fatigue. I had several Damascus barrels I kept where the chambers had blown to show them why these guns were now only wall hangers. . . and should NEVER be taken out into the field. They would probably do OK with black powder loaded shells, but they would chamber smokeless shells just fine. . . and blow up! No way should they be put in a position where that could happen accidentally.
A gunsmith can tell you if the timing and functionality of a Colt Lightning or Thunderer is working correctly without firing the gun, which puts it at risk of destruction. . . a badly timed revolver is a danger to itself and its user. My thinking was more on the antique flintlocks and cap and ball firearms. . . but also to any black powder antiques. If one wants to shoot them, use a replica. EVERY modern firing of an antique detracts from the value a bit. . . because it has to be cleaned and both shooting and cleaning are going to add wear and tear to the firearm and potentially destroy it. Why take the risk?