That would be a VERY good reason to stop the prop from rotating. If you hooked a line (even a crab-pot) and wound it on your pro, then tried to untangle it after that, then you'd really be in a pickle.
Good point, Blye.
I'm sure then you'd be better-off locking it off in reverse than having to challenge the transmission and rest of the drive train of your engine. At least the impact of the transmission would be a lot more gradual. Kind of like drinking, for example. ;^)