Re: Your style of playing a single player campaign in relation to diplomacy
I like having an ally when I'm going into a tough war, such as when there's a high risk of having to fights wars on two fronts and my ally can hold off the enemy on one side while I fight on the other. I like the way that regions held by full military allies count as yours for victory purposes. It can also be useful having an ally when you're a faction which can have cheaper mercenaries, and there are useful mercenaries in their territory.