The way i look at it fly, walk, telport, swim, or magic carpet or steed either way they get to the destination and have to dismount to continue the journey in the adventure.Good luck getting a mount into a dungeon or if you have to search the ground that is heavly forested and over grown with thick vines or thorns or foliage you will have to dismount.
It doesn't concern me how ppl ride port or travel to the site.
If someone worked hard and skilled up to learn how to tame such beasts and use such magic and then trained to ride and control it ..it doesn't bother me.
If it is a flying mount i am sure there is flying obsticles as well, wind storms, other flying creatures that could cause hazard.Not to meantion what does the fortress or town think of some dragon or hippogriff or strange monstrous mount approaching do they arm the balista or catapults and bow shot?
it all comes with whatever someone chooses to use and they have to deal with thier choices in game.