Shi... happens. Some for them, others against. Some will tell you no effect on the springs if you leave the gun cocked, some will tell you, you don't know what your talking about. Some have come up with ways to release the trigger without snap caps. Been discussed several times on this board through the years.
Every once in a whiles someone releases the trigger on club grounds when he thought he had snap caps inserted. I say he, as never read an article where it was a women that did it.
A death at a gun club a few years back, as well as a few injuries here and their.
Beretta warns of possible firing pin damage...just finished perusing my new 692 X Trap manual. I like snap caps and they are in every gun I own. For O/U just pop them in...fire both barrels...remove forend...then breakdown into case. If using an auto or storing in a long case it's just fire both chambers and pack her up. As far as springs go...they do get weaker...faster...if left compressed. Biggest problem though is the shorter "memory" your springs will have when stored compressed. They don't return with as much energy or length if stored for long periods compressed. You mileage may vary as to whether this actually leads to misfires from weak hits.