Yeah, AI should never plea for help unless it think it could lose. Like player. If it start loosing a war, for some reason it should just use CTA. It would be lot more logical.
Let's say that if AI got twice the number of enemy aliance soliders, it should not ask for help. For what??? Unless it starts to loose provinces and armies of course.
EDIT:
But on the topic. As strong country i tend not to have much of allies, as i don't need anyone else strong - i can manage wars on my own. But if i got strong enemy, it tends to be usefull to get an ally just to scare away the strong neighbour. For example i got allied france as ally, when i was aragon. I was also allied with brittany and sabaudy, and warned them just in case - to contain them as an ally, and to get excuse if they attacked someone i don't want to fight. I was not allied to castille, cause i did not liked an idea of helping them in any way. Whatever they were usefull - in their role. Castille never attacked me. So the same was with the other allies, to contain france - none got attacked by france. I also managed to vassalize algier-tunisia-tripoli, with tunisia being christian and rest muslim. Ofc. There is one ally that is allways usefull if it stick to you, so best way to make it always be your ally is to vassalize - Papal state! And ofc i did vassalized them. But i got corrupted save... FUUU!!!
So allies are usefull. But you just need to know how to use them. I mostly don't make aliances unless i need them to contain someone, or to destroy someone that is as strong as me or even stronger. For example i use to ally Teutonic order as moscow. They tend to be usefull, aspecialy if they come to help with lithuania. Or give you CTA in defensive war against lithuania or poland/lithuania - aspecialy if they got allies like HRE bohemia, that do help them in such war.
I mostly do only short term aliances, unless i offer them to vassal states. Long term alliances with AI, are useless, as they will bring you into wars you don't need or want.