I have had allies of mine aid me in battles. I took a province from the Egyptians with a Crusade. It was bordered by the Byz whom I was allied with. The Egyptians counter attacked with about 2k soldiers. I only had my small army left, and the Byzantines sent a pretty large force (about 1.2k) to help me out. Its impossible for you to predict and aid your allies in the first battle, only the AI can do that, but you can break sieges, like you said already. I have had many marriage proposals turned down by some of my closest allies, so I don't think they depend too much on previous alliances, and if they do then its not much.