That's a good point! Sleep and Color Spray type magic needs to know hit points.Yes, and that's why Flee Mortals includes a hit point rating that is compared in those circumstances. As quoted by Quickleaf:
That hit point rating would also be used for spell effects that check that sort of thing like sleep. It's a nice correction for that sort of thing.
I've got a question for you and @Shardstone - we're talking about suspension of disbelief around minions.
Does the part with MCDM minions where they take no damage on a successful save vs. fireball not break your suspension of disbelief? You could have a bugbear leader and a handful of bugbear minions next to each other, and if the bugbear leader fails its save it could be incinerated while the minions could succeed and be unscathed.