the inherent downside of health packs is this.
if you get caught in a situation where there are no health packs and you are quite low on health, but are at the start of a fight with two hunters or something.....youre slightly screwed unless youre packing halo 1 pistol
cod2 was the first game (from what i remember) to use the regenerating health system, and initially i was like 'wtf?' but it was quickly adopted as the main health system for all fps games since.
one recent deviation is in far cry 2 and moh airborne, where you have 4 or 5 'bars' of health, which deplete, and if you dont lose the entire bar, it regenerates. Interesting concept that, if properly employed, could make for a good compromise to be used in Reach.