Only the player can finish her forever. Mobs, traps, and anything besides the player just brings her to her knees. But the player can easily one-shot her when she's like that, killing her. Even with Unrelenting Force, I think.
So, lots of people kill her inadvertently.
It seems that most adversaries just ignore her when she's in her incapacitated position, but she can still be killed, and not just by the player. I've set her essential to stop any inconvenient killage occurring.
Talking of console commands, they're the only reason I know where she is half the time: compared to, say, Veronica (who seems very much her alter ego) in New Vegas, Lydia isn't terribly good at keeping up; either that or has a liking for wandering off randomly.