Rp reason because its more realistic. People do whatever they can to shorten the load on themselves. I doubt my hero would be any different and would love to tame a horse for making travels that much easier.
Non-rp reason speed. Mounts would logically would make you travel faster and many people aren't interested in the scenery as they use to be. I've map completed orr twice...I've seen it enough. Some classes don't get easy speed buffs for example mesmers. I have to make do with no speed increases (except a random one from a boon) because I play her greatsword and sword/pistol. I'm not going to swap to the focus just for the speed increase since it'd mess up my entire build. That is the only weapon/skill that has the reliable speed increase.
Edited by Fizzypop, 23 January 2013 - 02:58 PM.