I understand where you are coming from as a strategy as simple as sitting on the roof would negate the ability of the NPCs to push a person and make them easier to take out from a safe position, however the NPCs would also bring additional loot to the situation so even if you only got ladder hatches, cameras and attachments from the bradley crates, there'd be a chance of better loot from the NPCs like MGLs, 40mm HE grenades and even the slim chance of a L96.