The same reason why Metroid has a gun on his suit.
But Metroid was only in Halo 2 and 3, and in Halo 3 he was player 2.
At least they had a proper player 2. It doesn't make sense to have four Optimus Primes running around, especially when they could have just used our multiplayer Autobots like they did in the last game.
Side note: Any word on when Bungie is going to fix that glitch where the vehicle modes only work on certain spots of the map? At the very least, are they going to fix the glitch where it shows the vehicle mode when nobody is playing it? it's quite annoying in multiplayer because I can't always tank.
I find it amusing that something a drunk person babbled at me is considered "common sense" by Dismind! - Brain Hammer