In the two years I've larped, and the three chapters I larp in, I've only seen real locks made to pick used once ever, and I couldn't even pick them, because I was NPCing. Traps, both disarming and arming, has come up maybe like 8 times. Other misc things that require ledgermain, have happened like 4-5 times. If you're a low level character, I recommend not taking legerdemain. (unless you are a hobling rogue where it costs like 2 build.)
That being said, when legerdemain is needed, and you're the only one who can do it, you'll feel like a hero. Also lockpicking is a pretty fun hobby anyway.