1. You can inherit some of his provinces if your ally daimyo dies without a heir (if you we're his first ally).

2. An ally, will attack his ennemy before you, giving you time (You still have to check your borders though, any daimyo, even an ally will attack a weak border province). With MI, if an alliance is broken by an invasion of a province, the daimyo who was the culprit will loose 1 point of honor.

3. You can gang up on a single ennemy, stuck between both of you (in my last game Oda my ally and I we're both attacking Takeda stuck in the middle, thus an easier time to conquer provinces).