It's a great skill! When I play a thief, I use it all the time to pacify levels, locking off all of a dungeon except the path between the stairs. For example, you don't want to be leading Yriggs back to the healer and find yourself stuck in a corridor with a hostile monster in front of you and Yriggs behind. The skill can prevent that scenario from happening. It's also good for caging cats, keeping monsters from picking up your stashed items, and so on. Yes, it's more useful in the early game, but so are a lot of skills.
Now I agree that between buying Open Locks and buying a ton of keys, it's a toss-up. But open locks isn't useless, and neither is disarm traps.