There is a mechanical door release if you're trapped inside. To get in from outside obviously needs the vehicle to unlock, so it has to be jump started.
Even if there was some kind of back-up mechanical lock I can't see anyone carrying around a key only for this specific eventuality. A glass breaker key-ring might be the best option — along with understanding how to use these emergency features in case you need them. A glass breaker might also save you in a fire or ending up underwater.