No one in either of the guilds (Beta / Phyll) & (Zeus(4) / Clarity) I'm in seems to find ANY benefit associated with having a Mount. In fact, after having one on Beta, I've chosen to not bother on Live.
o Damage given is not increased with as opposed to without.
o Damage taken is not increased with as opposed to without.
o Speed (really?) is not increased with as opposed to without.
I don't get to "The Lady Guinevere" on horse any quicker than anyone on foot.
But, I've finally figured out a way they can be of benefit. Grinding. Reduce the Grind Time 10% for each level of mount you ride: 10% for White Horse, 20% for Black Horse, etc.
This makes sense. Historically, mounts make you more effective attacking and provide defense against attack. They also get you from opponent-to-opponent MUCH quicker than on foot.