There are numerous risks that could be used to counterbalance any bonuses gained from riding mounts as well. Being on horseback could leave a player vulnerable to fall damage multiplied by speed, stun-locking them if they were thrown off, the duration depending on movement speed and weight. The bigger you are, the more goods you carry, the harder you fall. Riding on a mount could also create a larger aggro radius, attracting certain mobs such as cyclopes, griffons, etc. Predatory creatures in the world with a larger appetite for such things. This could even prove to be a useful but risky tactic for small raid encounters. Send a ranger out on horseback as bait, with track up. Let them lure the beast towards the raid, and maybe the ranger/mount lives and maybe not, but so long as only one member dies on incoming, ultimately it's worth the loss.
Jboots were great and everyone wanted them, but one could manage without a pair. That seems to be as good an example as any. It could even be that some buffs/mounts only increase movement speed in certain climates/environments and in others (icy) you wouldn't necessarily want to move faster, especially if unfamiliar with the zone layout. Why would spirit of wolf increase your movement speed under water? Wolves are good swimmers, but by no means as fast as when on land.