In the Halo 2 and Halo 3 Campaigns, it is possible to give powerful weapons to NPC allies. This is an excellent tactic because such allies never run out of ammo—they can fire the weapon limitlessly, and if you take the weapon back, it will have just as much ammo as it did when you traded it. Allies also do not need to reload weapons, this is a huge advantage when an NPC has a rocket launcher, thus reducing the time between shots. The technique is often used with powerful weapons, including the Rocket launcher, Sniper Rifle, Beam Rifle, Fuel Rod Gun, and Spartan Laser.
In Halo 3, giving NPCs "power weapons" is an excellent tactic for higher difficulty levels. As NPCs never run out of ammo, you can use an NPC sidekick to augment the strength of your vehicle—simply give a power weapon to an NPC sitting in the passenger's seat of a Warthog, Mongoose, Prowler, or Hornet. You'll have an NPC that can fire the weapon nearly-at-will (barring the "refresh period" between volleys) while leaving you free to drive. The Campaign uses this by default on The Storm; at one point, one has the opportunity to drive a Mongoose with a passenger in the back seat wielding a Rocket Launcher.
However, one should take heed in giving power weapons to allies, for on occasion, you will may be harmed by your ally, either by standing in their line of fire, or the AI for the NPC does not realize that you will be in the impact radius.