The problem with dry firing without something in the chambers (fired cases, snap-caps, etc.) is that if the primer doesn't stop the firing pin and soften the blow, something else in the system will have to. In older S&W revolvers it was the hammer hitting the frame, when the “hammer nose” (firing pin) was attached to the hammer. As I’m sure all of you know, in the model 642 and similar the firing pin is mounted in the frame, and its retracted by a small spring, that you do not want to mash.
While damage may be improbable, it is far from impossible, and in a defensive weapon the Old Fuff has enough to worry about, without giving Mr. Murphy any extra help. Snap caps may or may not be necessary, but they are excellent, and inexpensive, insurance.