As Napoleon said, God save me from friends, and I'll deal with enemies myself. And what's the point of an ally – just to get in the way and take away your resources? In a custom map, they could simply program its behavior, limit its buildings, and restrict its movement radius from the castle.
2. An ally is for fun – to watch the computer wars. You don't participate in the process to the maximum, but simply observe how it develops. You help out here and there...