I think you misunderstand. It doesn't matter if the BBEG kills one or even 3/5 PCs or even TPKs the group. He will still have very little use for the reserve healing feat in most campaigns because he will either not need healing enough for his action to be profitably spent on healing a little bit instead of killing PCs a lot OR he will need a lot of healing in which case the reserve healing feat won't cut it and he needs whatever the highest level healing spell he can cast is. The feat solves a resource management problem that bad guys simply do not have vis a vis the PCs.