With hit point thresholds for spells like sleep, I don't think the cases in which rolling monster hit points will add more variety will be that niche. As it stands, dropping a sleep on a group of fresh orcs will just make them all drowsy. With random monster hit points there is a chance that some of them will need to save or fall unconscious.
I had not really realised that, and your right. Hit Point values are a lot more important then it used to be. I may have to think about this.