I was at Blue Mountain a couple weeks ago and I was estimate that 95% of the board there are left unlock. It's a thieves paradise.
I bought my wife a ski key for Christmas and we are able to lock up 2 snowboard with 1 ski key.
IMO no lock is 100% secure, all locks can be cut off, etc. However, when there are so many board left unlocked there is no reason for a thief to go through all the trouble to steal your locked up board.
If your hill has racks that take a ski key, I would pick up a ski key.