From my experience I feel that it is far, far harder to control the compression of the spring with something like pliers, of any type, which move the spring quickly with great leverage. Plus, if it does fly out of the pliers after removing it from the lock and flies across the room (or into your face), there is a great chance of breakage. With the proper spring cramp the thumbscrew slowly compresses the spring in a very controlled manner. I've done it both ways, and the greater control from a mainspring vise gives me much more confidence.
JMTCW, SRH
