When ever I used to park my mini in a dodgy part of London, I too would would remove the rotor arm from the dizzy and carry it around in my pocket. I figured that even if any "would be theif" did work out what was missing, they would be very unlikely to be carrying around a spare rotor arm with them (unless of course they were also a mini owner who was using the same trick to prevent their own mini from being nicked!)
A little hint though, if you do try this one...... If your bonnet is the internal release type, then dont place your car keys safetly on top of the air filter housing/box of your locked car, whilst you remove the rotor arm from the dizzy. It's way too easy to slam the bonnet shut with the keys still under there and no way of getting to them!! (DOH!)
Because your not letting us learn from your mistakes
Thats sounds like the sort of thing i'd do, lock the keys inder the bonnet whilst tring to remove the rotor arm from an electronic ignition.
(Don't worry i do have some mechanical affinity)