What doesn't make sense in Halo 2, is why there's no kevlar in the Mjolnir armor. After your shield depletes itself, it's bang and you are dead.
I thought HL 2 made sense, because it's a hazardous enviroment body suit, that depoisons you, enhances performance and provides servo functions and yet doesn't weigh much of anything. I think it makes quite a bit of sense in the pseudo-scifi setting.
About CoD 2, well I thought the regenration merely represented you taking care of your bleeding and resting for a while to catch breath. Yes, it's stupid that there's no setbacks in taking damage, but hoarding health kits or tugging and protecting a medic level after level would have been much more gamey. No to mention awkward and focus-shifting in the case of the medic.