I buy expensive ammo to carry because of the effects it will have after shooting at your intended target. I don't want a bullet going through my intended target and possibly causing harm elsewhere. Ammo technology has changed a lot in the years and I'll take every safety precaution when loading my firearms for self defense.
For personal and home defense I like to have hollow points to reduce the chance of something going through my walls and out into the general public, or to keep my bullet inside of my intended target and not through them and onto something else.
For the winter I will switch to just a FMJ range round or make sure I'm carrying a +P round since everyone is walking around with so many more layers on.
If SHTF, you're right, use whatever you have laying around. For range time I just use the cheapest FMJ round I can find.