I like the idea and i support it.
The rarer mount are different animals or something like that, so if u want something complitely different u can buy them. If you can't just afford a rare mount, the game will have money entries for the mount scroll usual for NM or Hm as well. You will always have an horse or a leopard or a lion, but i woul like to buy something to make it as much personal as i can.
Just my opinion ^^