I was thinking about that Mike!
I am a weapons tech. I cant say that I have ever found a fault with a pistol that I could say was due to letting the slide fly forward under spring pressure without any ammunition in the pistol.
The forward action of the slide on a Browning HP ends as the slide locking lever pin cams the barrel up into the locking lugs. The strain would be to the pin, and to the locking lugs. I very rarely see any trouble with these areas, even given the age of the pistols. (1942 manufacture)
If damage has resulted to the hammer and sears of other pistols, I dont understand how yet. Aside from striker fired pistols, they all engage though their own spring's pressure, and not through any direct action of the slide on the hammer or sear. So the force that this operation happens with would be relatively constant and set by the force of the sear spring and hammer spring.
I always thought that this was an etiquette point myself. Controlling that slide's forward action clearly demonstrates that you intend to take good care of a pistol someone is letting you try out. I wouldnt want to recommend anyone do it with a CF weapon though. It would be a bad habbit to get recruits (or anyone for that matter) into, just to have to break them of it again when they proceed to do it all the time.
When we talk about damage to firearms resulting from people letting the action fly, the only thing that comes to mind are the innumerable C6 and C9 machineguns I have had to fix because somebody saw fit to conduct dry fire training without the bolts! (This bends and chips the firing pin and/or spring as its a part of the carrier and not the bolt itself) It keeps me in a job, but its a sure way to screw your entire platoon's MGs!