To prevent infinite ammo, aka being OP.
Have you realized that they let you use pikes and cabal tank for a few minutes but the later stage forces you to get off the vehicle because they cant go through? Because they dont want you to kill Valus or Aksor easily with those.
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You're already prevented from summoning your Sparrow in those areas, so I fail to see why that would force them to remain unarmed. Sure, you could get one in to Valus, mayhap, but the relatively narrow confines of the tank interior and the bullet-spongey nature of the Cabal would really limit the amount of vehicular firepower that would reach him.
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Good thinking, I did not think that through, when your in a darkness zone it should shut down, like landing gears on a plane, (Like Fold like landing gears) and the ammo would be from the higher level enemies, like a reaver vandal, or a hollowed night, rarely from normal enemies, you could only buy the ammo from your faction, FWC, New Monarchy, Dead Orbit, and Crucible Quartermaster.