Sorry to add more without having originally said this in the post I just made, but, this would also allow room for experimental weapons. One very common complaint about the Forerunner weapons were, they weren't alien enough. Oh, a reskinned Shotgun. Oh, a reskinned Rifle.

Imagine in Campaign us actually having original fun weapons, like what the Needler and Plasma Pistol were. Alien weapons that we had to briefly take a moment to learn how to use. Because of "how is this going to work in Multiplayer?" They get messed with in ways that ruins what they are.

They don't have to be default weapons in an arena map. With that, they don't have to be balanced.