I usually ski with a friend and we will trade skis pairing one of mine with one of my friend's skis and then put the unmatched pairs in different locations. I always figured this would keep someone from accidentally taking my skis and also deter a thief because: a) an unmatched pair is not very useful and b) looking for the other set of unmatched skis would be a pain. Am I fooling myself that this system works or should I get a lock?
I think you are fooling yourself. It isn't like most people haven't seen this done and the other set is never that far away (maybe a couple of racks). Locks are cheap in my opinion, most skis are not.