While the idea of living mounts kinda fits with the whole mythic sci-fi vibe, I get the impression that, like our guns are our swords, our spacecraft and sparrows are our viking warships and our valiant steeds. Horses seem a little too vulnerable to the sort of weaponry and enemies we'll encounter in the wilderness of Earth alone to conscript them into battle. As far as getting around the city on one, I guess, but it stands to reason that there'd be more practical ways for the adventurer/warrior classes to get around that would be less of a hassle to maintain. I mean, we already have those speeder bikes, and the idea of having some still standing freeways around the city to go bike racing on in lieu of fast traveling seems much more compelling in terms of gameplay possibilities. As far as non-combatant NPC's using them, maybe out on settlements for agriculture and transport, sure, why not? Makes sense with the setting. There's also the potential for exotic alien fauna being used as mounts offworld to consider.