Why aren't more HEV suits being produced to give the resistance fighters an edge in battles? Why is only Gordon using the gravity gun if it has such a wide creative usage in combat? My point is that some items are there and only available to the player because they're fun. Yes, it would make more sense to see those being used by the other NPCs, it would establish more consistency of behavior, but it's a not a must.
And it was supposed to be a spin off, something not as serious - think of The Zombie Island of Dr. Ned for Borderlands.