From what I've read here, the AI doesn't have the best reputation as an ally. I've accepted alliance requests from decent-sized Castille in the hope that we could pair up and contain France, just to see Castille getting steamrolled as if they actually wanted it. But aside from the OPM vassal swarm that hunts down rebels and carpet sieges, sometimes the AI ally is actually useful. I'll share an example, then please tell yours!
I was playing as Sweden in the grand campaign, and fresh out of the PU with Denmark I was looking for friends in a hostile world. Being the mid 15th century, I was ofc poor as an otter. Denmark and Norway was allied, and Norway was allied with Lithuania (bordering me). Denmark wanted my main guy dethroned, I wanted Norway subjugated, and Lithuania wanted some casual spoils of war. (Norway probably just wanted to get away, but tried to do the best out of the situation.)
Then I found the Hansa, or rather, they found me. They stuck around no matter if I was the aggressor vs. Norway or if Denmark or Lithuania smelled weak prey and declared war on me. I couldn't afford an army big enough to effectively fight a two front war, but the Hansa was always willing to join without me even having to ask. While I took care of the Lithuanian business, the Hansa prevented the Danes from crossing the strait, or fought back the Norwegian army and took back the Swedish provinces that had become occupied, until Lithuania had peaced out and my army could return to the main theatre.
This went about for four or five wars, until both Norway and Denmark finally were vassalised (this was IIRC my second EU3 game, not much skillz from my side). I don't recall if the Hansa ever got any material gains, but I guess a true war monger isn't disturbed by that!
I was playing as Sweden in the grand campaign, and fresh out of the PU with Denmark I was looking for friends in a hostile world. Being the mid 15th century, I was ofc poor as an otter. Denmark and Norway was allied, and Norway was allied with Lithuania (bordering me). Denmark wanted my main guy dethroned, I wanted Norway subjugated, and Lithuania wanted some casual spoils of war. (Norway probably just wanted to get away, but tried to do the best out of the situation.)
Then I found the Hansa, or rather, they found me. They stuck around no matter if I was the aggressor vs. Norway or if Denmark or Lithuania smelled weak prey and declared war on me. I couldn't afford an army big enough to effectively fight a two front war, but the Hansa was always willing to join without me even having to ask. While I took care of the Lithuanian business, the Hansa prevented the Danes from crossing the strait, or fought back the Norwegian army and took back the Swedish provinces that had become occupied, until Lithuania had peaced out and my army could return to the main theatre.
This went about for four or five wars, until both Norway and Denmark finally were vassalised (this was IIRC my second EU3 game, not much skillz from my side). I don't recall if the Hansa ever got any material gains, but I guess a true war monger isn't disturbed by that!