Don't need a set of croppers, an allen key undoes the disc and the bike can be wheeled away, as the lock rotates as far as the caliper and the wheel then turns reasonably freely if the disc isn't too corroded to the hub.
Bike thieves generally come as more than one person and can lift the front end of a bike easily and wheel it away whether you have a lock on it or not I'm afraid.
The scrotes that nick bikes for their own use prefer twin disc setups because they can secure the disc with the lock attached and still have a brake to play with as the other disc is still attached to the hub.

Pete.