AI reinforcements can be useful in seiges if you're sure you have enough spies that the gates wil be open. That pulls part of the enemy army away from your own attack, and (assuming favorable odds) gets it over with more quickly.

Sometimes I enjoy a nice carefully planned seige, where I want to take minimal casualties.... and those I do alone. But sometimes I just want to end it quickly to get on with the next objective. If the gates are opened by spies, then the losses suffered with AI army reinforcements aren't as bad as just auto-calcing through a seige.