Stashing locks has become step 1 when arriving at trails here in NY. It's new to me (bringing a lock mountain biking) and I think it's unique to the NY MTB scene. Maybe it's more unique to the "Car-less NY MTB" scene. At any rate, a lock is 100% necessary and comes into play when you need to run into a store or fend off crazies on the train. The other side of bringing a lock is remembering to pick it up when you leave.