Quote1. Are animals useful in combat? Do you use them? If so, why? If not, why not?
I use raptors from the Dinosauria mod. I avoid using livestock because I am overly cautious about losing them, even though a lot of them breed very quickly.
Quote2. Are there annoying/weird points about animals in combat?
Friendly fire is a major concern later on, so animals rarely play a role in large raids. Having newly trained pigs and llamas wander into combat because they were assigned a master can be pretty annoying--I have to open the animal tab and reset a bunch of animals every time a raid starts.
Quote3. How are you using animals in combat?
Fast animals (e.g. raptors) close distances very quickly and are better than armed melee humans. I also use them to engage archers as quickly as possible, and to chase down kidnappers and fleeing raiders.