I've noticed every pet can outspeed a sprint-jumping player, except for the bat. Perhaps speed needs to be tuned? Also to be fair, it's difficult to left click me while I'm riding the Wither, so perhaps if someone's pet gets left-clicked, it could flag them in combat without dealing damage or something so that way, pets are rendered actually useless and forced to be only cosmetic. I recall
@spiralz not able to properly backstab me while I was on the wither.
When I was a shaman roaming solo, healing totem, healing surge and cleanse spirit could all be cast from the back of the wither without any drawback to speed. This made for PERFECT escape tactics, as it was difficult to flag me for combat while I just escaped and healed to full. The same goes for the warp skills, they're usable while being able to outrun one pursuing you.
While we're on this topic of pet riding, I've fallen into lakes of lava in the Nether and only had to press the wither mount macro I have (which is /pet wither|/pet ride) and I'm essentially able to coast across a lake of lava (slowly, of course) without being set aflame.
Perhaps pet riding would need to be reworked? I do enjoy having it, as I'm a freak for cosmetics, but I also agree it is unfair. I've lived more times than I should have in the midst of battle.