I've seen the AI have trouble when there is no direct land route to where their armies would be needed. But if they can just walk somewhere, they're fairly useful and effective.
Helping them out in turn can even be a good thing too. I once helped the Danes agains the Hansa and got two provinces out of it, while they got the aggressive expansion penalty.
What is clear though, is that for each stack, the AI decides if it's used to siege or to hunt, and after that doesn't really change it's mind so it will happily siege with a stack of 20, while enemy armies of 2-3 run circles around it.
An ally in hunting mode with a big stack can be very useful though, you can just siege and save your units, while his army kills everything and suffers the casualties.