The game neither forces you nor requires you to have a skin on any mount. All the mounts do function in discribed manner without skins just fine but do require a somewhat skilled player and patience at times.
Besides tons of FF XIV players also do glam runs in order to just obtain a certain glam or suffer or enjoy through savage content only to get a specific mount (axolotl for example). No actual use in both imo when my armor is already decked out and max ilevel.
There always will be those who spend money on games (except for subs or the base game) and those who rather invest good amounts of their free time which could be used to go out with friends or other stuff.