I would think it's obvious? You have limited horse resources, and further limited by logistics. Certainly, those limitations becomes less of a concern after a certain point in game when you have expanded your faction's territory and built enough cities to field more than one stack of doom, but until then, you have to make do with the limitation. I like archers because of rain of arrows area attack. Units who don't even need to move to inflict damage on the battlefield is always a perk. Given them initiative bonous buff items and they can really soften up the opposition, allowing your melee  units to simply clean up. Pikemen are great against many creatures and other riders. I always try to have a couple in my army stacks. 

Snipers are pretty useless though. I just haven't found them to be useful at all even when I've spent the time to level them up.