Personally I like to do "in-betweens" to blend between two different looping animations. You might not need this for everything though.
You don't know how long people will stay crouched for, so if you want your character to move while being crouched, you'll need a looping animation of that part.
Using Spine to "calculate" the crouching pose, might be hard and probably not doable for all characters, but its worth trying it out I guess. I assume you're thinking about procedural animation here.