Today, damage to firearms from extensive "Dry-Firing" typically manifests itself in several ways that include broken firing pins, cracked breech faces, and extra wear to lock parts.
Firearms are primarily designed to be shot with live ammo; not dry-fired, aka "snapped".
The various parts of firearms (like all machines) have a service life and expected "mean time before failure". This expected service life varies greatly among brands and models. Often, the practical service life of some guns is surprisingly short; a few thousand shots or less; while other guns seem to be good for well over 100,000 shots.
While "Dry-Firing" (with "Snap-Caps") stresses a gun much less than firing it; it still cycles and eventually wears the parts involved in the "Dry-Firing" cycle. On a self defense "carry gun" extensive "Dry-Firing", over time, can leave the gun appearing to be in excellent condition when in fact it is about to fail, potentially when you actually need it in a real self defense situation.
There are various and often conflicting opinions as to the actual benefits of "Dry-Fire" Practice. It seems to help some persons greatly; while having little or no measurable benefit for others. If it improves your shooting, it is much less expensive than shooting live ammo, as long as you protect your gun with "Snap-Caps".
Most well designed, good quality firearms will tolerate numerous "snap" cycles that are actually necessary for various purposes (other than extensive "Dry-Fire" practice) over the firearms expected service life. The manuals that come with some guns discourage "Dry-Firing" beyond what is actually necessary for non practice purposes such as dis-assembly and cleaning.
Sometimes "numerous" may be several thousand cycles or even significantly more than 20,000 cycles before problems arise. Sometimes numerous my mean fewer than 1000 snap cycles (especially for some older SxS shotguns fitted with one piece hammer/firing pins that are notoriously prone to breakage) thus creating the need for and the invention of "Snap-Caps" to de-cock the hammers for storage.
A firing pin or striker is primarily designed to strike a fresh primer (usually made of brass, aluminum alloy, or very soft steel) with its nose; and be decelerated (cushioned) by the deformation of the primer. A coil spring backed, brass cylinder striking surface, "Snap-Cap" closely simulates the firing pin nose striking a real primer; thus preventing undesirable stressing of parts involved in the firing cycle. {Unfortunately, in the USA, this type of Snap-Cap has largely been replaced on store shelves by solid metal "Action Proving Dummies" with a piece of polymer material inserted at the primer location.}
Most firing pins/strikers are not really designed and/or intended to take the deceleration load anywhere but on the nose (end) of their structures. Most firearm parts that surround or interact with the firing pin/striker are not designed to be repeatedly struck, hammered, etc. by extensive dry firing or snap cycles without using coil spring, brass plug "Snap-Caps" that are usually made in Europe these days, and are still available from Tripple-K Mfg in San Diego, CA.
Going back up this thread to posts #8 & #10; I personally would opine that "Snap-Caps' should be used if one frequently engages in "Dry-Fire Practice" sessions. As to using fired shell cases as substitutes for "Snap Caps", they are initially some protection; but quickly become ineffective with repeated strikes. If using a spent .22 RF case one will want to frequently re-position the case so that the FP is actually striking a fresh spot on the rim.
Hope that this is of some use.