Lydia "use" to get whacked early in the game--that was until I started letting her carry her own health potions. She will use them too, besides, it's hard to use "heal" on her when you're getting your own clock cleaned. I started to give them to her around lvl 18. I'm at lvl 71 now, and haven't lost her since. If I do see her on a knee, then I know she ran out.