The whole point of an instant healing item is to save your own life.
If they don't restore enough hp from low health to survive even one more thing, then they might as well heal no hp and just be removed.

If they really don't want people relying on them so much just make their cooldown really long, like five minutes or something.
If they're being hit by more avoidable stuff than that then they're probably tanking the floor most of the fight anyway.