Not many topics on this, so I'll make one. Halo 3 ODST's stamina is just a reskinned Spartan Energy Shield. Bungie could have used the old-school armor system from first person shooters such as Turok The Dinosaur Hunter, Goldeneye, Perfect Dark and Timesplitters, where you have armor packs and you pick them up after the bar depletes from hits. Seperate from health packs by the way. Orbital Drop Shock Troopers are supposed to be different from Spartans. Why didn't they change the armor system?
Because they wanted the system to remain familiar. I agree it would make more sense to have pure armor and health pickups like in Wolfenstein, but I'm guessing they thought it would alienate a lot of their player base.