It is bad practice to run them into the ground. I am sure you know that.
No. It is a really bad idea to run any of them to totally flat.
Towards the end the weakest cell in the series chain gets wrecked by the others and will never properly recover. Deep discharge will tolerate a bit more cyclic abuse over its nominal Ah capacity though. Car batteries are really only happy delivering very high output currents for shortish periods of time. Run them flat into the ground and you are asking for trouble with predictable results.
As someone else said protection circuits are offered to drop the load when the battery voltage is getting too low and there is a risk of not being able to start the engine.
That or a low charge indicator flashing led to warn or impending doom.