You can buy other things with a bi-colored stones.
Minion's, Orchesterion rolls, crafting items to sell.
I bought stuff like this and still needed to farm more to get everything after I reached everything on rank 3.

Also, you are already flying at max speed. The extra thing is just for faster ground speed.

Oh, and when you have the spare Gil then go ahead and buy the mount. It cost me just 55mil Gil.
And it is a good way for other to farm money so that they can buy whatever people need Gil for in this game.