: It must have been a deliberate decision on Bungie's part, perhaps to maximize
: the contrast with the next level, which essentially has unlimited ammo.
I should correct you on that. For me, IIHARL... is one of those levels where you have to do a lot with a little. Two Shotguns and less than 60 shells? Sure I can blow those Troopers and Hunters away, but by the time the first door even opens, I usually have used up a great majority of the Shotgun ammo provided to me. Once you've got the Rocket Launcher, it's straight shooting, but you have to get to it first. They don't even give you an Assault Rifle, much less a Fusion Pistol!
Now, if you're playing through from the previous levels, you'll pretty much invariably have massive loads of ammo to spend, allowing you to play it lazy and just pour lead into the enemy. But in the meantime, the scratch-starter has to handle dozens upon dozens of enemies with minimal ammo. Basically, both of these levels seem to be deliberately forcing you to suck away your ammo. Now put Sorry Don't Make It So on the end (which has nothing more than Alien Weapons and a Flamethrower), and you'll be Begging For Mercy. ;)