At most competition pistol matches, shooters are required to "unload and show clear" at the end of stages. This entails removing the magazine; pulling the slide to the rear if it's not already at slide lock; physically showing that the chamber is empty to the range or safety officer, letting the slide go forward; and lastly, pulling the trigger (dry firing) the pistol before re-holstering. I don't think inserting a "snap-cap" in this process will be well received.
My personal dry fire practice drills would be cumbersome with "snap-caps."
I would also like to hear a technical explanation why dry fire with a Glock, other than for disassembly, should be with "snap-caps."