You evidently have never had a bad ignition switch/lock cylinder.
Had it once - I had to carefully position the key or the fan would not turn on.
However, a switch that does not turn off the engine by itself is an annoyance at worst. Also, I would get it replaced quite fast if I had that problem. It would be a problem only if it failed at the same time that the throttle decided to stick full open. But even then there's downshifting etc.
You may have also not enjoyed the phenomenon of dieseling where the engine continued to run on its own even when you shut the ignition off.
At least in my car (and one other car of similar age) dieseling does not provide any real power (barely enough for the engine to turn) and can be stopped quite easily (either step on the gas or put it in gear).
Because of the LPG mod, I also have one m ore way to turn off the engine - if I am currently driving on LPG, I can just set the fuel selector to "none" and the engine will stop. With gasoline it's not so easy, as when the fuel selector is set to none, I still need to wait for the fuel in the carburetor to be used up (the reason for a three position fuel selector).