I'm still relatively new but I wanted to give an opinion on the mounted thing. Teleporting I can't see a reason why but talking to NPCs I may have an understanding.

Imagine you've around a popular NPC and everyone is on their mount. Some of these mounts are HUGE. It would end up being very confusing to even find the NPC in those sorts of situations.

Regarding the market....what I find irritating is that while you have to use your retainer to sells tuff, you have to go back and forth between your retainer and the market board to see what you should be looking to sell your items form.