Guildhest: basically a playing 101 for basic situations, i can see how it can seem useless now that you plow through low lv content.
potions: there hasn't been a moment when i see my healers of endgame static not using mp/status potions even poison pots helped clear t13
Food: again i dare you do coil/alex savage without food and try to clear it before nerf.
Treasure Chests: I use the drops all the time to sell/desynth/gear up/get grand company seals.
Chocobo: Instead of asking help all the time when people are busy i use chocobo to do some things fast.
Hunting/sightseeing logs: they are kinda of point of view from developers that from that point you can see something beautiful they have created and i've got some of my favorite screenshots from them
City Airship: yeah people with very low gil use them but for me it just there to make it feel a bit more alive. (the game)
Linkshells: lets see... i have my fc, my ls for closest friends, my ls for alliance between my fc and another fc, my ls for fishermen who help each other with info, and my hunt ls that sends me news of hunts... now i ask how can you not find a use for it?
Irrelevant Quest lines: made because people ask for more content which becomes abandoned as another fight until someone bored does it again in lv cap to remember how challenging it was originally and if you erase it people would say this game has no content or diversity and everything old disappears and leaves nothing to do when you want to rechallenge yourself.