I will admit, it's been a little challenging to get my helmet(s) under that seat and dangle well... "locked".
That said, the lock "handle" has been way to easy to pop open and I figure if someone is going to take my helmet, they're just going to cut the strap trying...
I just can't trust leaving it on the bike unless I can see it at all times.
Since that handle has been so easy to wiggle or pull open, I don't even store my cable there anymore. I carry it in a backpack if I'm going to need it.