If Bungie does decide to add AI into the Custom Games, then it would be literally perfect for Machinimas.
Umm... no, actually. AI. Artificial Intelligence. They're going to do whatever they want. Namely kill you. Not pose in awkward positions and places for a three-second movie shot.
You never know if Bungie will make them listen to you. i.e When you're about to spawn them, you can choose whether or not you would like them to be hostile, or whether or not you would like them to be standing still, Red Team, Blue Team, etc.