The problem I see with using hit points to govern other effects is all those other effects - spells that used to bypass the hit point attrition - just become additional ways to bash down the opposition. I want an alternative to that.
I understand, its a big part of how D&D traditionally views the mechanics of magic. But honestly...why? The whole "casters do it different" thing is part of what makes the whole balancing act more problematic.*
HP are supposedly this big abstraction that amounts to: "Your ability to keep fighting" or perhaps more accurately "Your ability to not be taken out of the fight." Sleep damage would just represent a reduction in that ability due to magically induced drowsiness.
All damage can then be viewed as simply "the ability of this attack to take a target out of a fight" and "any ability to take a target out of a fight is damage." Differences in vulnerability between targets and attack types can be represented by special saves (or NADs), resistances, damage reductions, and immunities.
Maybe effects like sleep could target an alternative condition track (a bit like Star Wars Saga Edition). Sleep and other effects could knock you a couple of steps down that. If you wanted to incorporate hit point attrition making a character more vulnerable, hit point damage thresholds could take you steps down the track as well. But some effects could target the condition track directly.
Condition Tracks are one of those things that I love(d) on paper, but not in practice. I'd much rather make a note in HP-tracking box: "8 Sleep" and know that if I get them back when I wake up. Rather than having to establish and know what several condition tracks do (and how they interact), I get a fairly easy-to-interpret "sleep", "poison", or heck "petrification" damage, and only when needed.
I'm not a big fan of having HP in general, I think its a lousy system (especially when combined with D&D's traditional healing magic.) However, I haven't found one that I particularly like better. Given that we're going to be riding this sacred cow, I think we might as well accept them as a combat pacing mechanism and be done with it.
*Charm, Illusions, and some similar related effects, are one area that I don't think are modeled well by dealing damage, especially out of combat....but then, I'm not sure these spells should work
during combat, either.