Why did you never leave a baby behind?
I have never needed to 'heat' a formula bottle. And mixing requires shaking which can be done anywhere. I never felt one was easier than the other, just that it's annoying if you forget to BRING the formula, but you really can't forget to bring the mother if she's taking care of the baby.
Because I felt the baby's place was with me. Anyway I didn't have any grandparents to leave my kids with; only friends, and it wasn't fair to leave a baby with a friend as it is so full time. I only left kids who were in school or nursery.