I don't think that is 100% true. If what you are saying then how would one turn on the four way flashers, lock the car on the side of the road and proceed to get help if say you ran out of gas ?? The interior light, horn, cigarette lighter, headlight and possibly a few other accessories will operate without the key in the ignition. So to answer the OP's question, it's not a bad practice to have a quick disconnect switch and to use it. Just my thoughts.
