HP are only luck in the sense that "you are lucky that (arrow) didn't kill you". Which is why experience allows you to create more "luck" - you get better at knowing how to turn at the last moment to take the arrow on your shoulder, where your armor is thickest, and it cracks off only leaving a bruise instead of collapsing a lung.
It IS luck, but it is (IMO) better described as effort. Things are just happening so fast, that you are lucky that your efforts pay off and you don't get yourself killed. Some of that is also "grit", in that a lesser person might give up and die, while you suck it up and carry on.
I (respectfully) don't like the OP's suggestion regarding spending it to avoid dying from poison, because that is already what is happening when you take 12 poison damage but have 14 HP. If the poison is enough to kill (drop) you, it should do that. You shouldn't be able to pay some lesser amount of HP to avoid it. (Unless, you know, you have poison resistance, but again, that is already accounted for).