For someone to steal a derailleur they would need to remove the chain, shift cable, and unscrew it. Thats a lot of work for a derailleur.

I simply dont have components that would be worth such an effort to steal so its never been an issue to consider.
When my bike is hanging next to a dozen others on a trail, its almost always the oldest technology. While i consider it to be better than some current tech, there will almost always be bikes which appear more appealing. Same deal for components.