First of all, what you're talking about can absolutely be done, I've done it myself just once in the campaign where I'm a player, but only once because I like to treat my spirits with respect.
Second, the various reasons you mention under the " why npc's would do this " are the opposite of "treating the spirit with great reverence ", it is talking something amazing and making it trivial and I would personally consider it demeaning to the spirit as well. Remember that spirits are sentient creatures and I don't see many of them liking being made to carry spoiled rich brats around on their back.
So if you want to treat the spirit with reverence and still use this ability I would say, use it sparingly and only when nothing else will work.
Also if I remember correctly there's precedence in the fiction, I believe Talons ally spirit had an alternative form which was a motorcycle.