Okay, you have to differentiate here - in RDR there was focus on riding a horse - okay, the horse could be detailed.
In M&B, if we're talking about riding a horse, it's riding a horse in front of dozens of metal-clad knights shouting at their enemies and waving their swords, lances and maces. If their mounts look only okay-ish, but allow smoother battle, than count me in as the static-mane believer.