It's really been a long while so I can't remember if those old level 50 quests let you turn them in as another class, I'm sure class and job quests don't because they're obviously class specific. At least in 1.X you could turn in level 50 quests with lower classes, leveled a lot of crafting classes just killing primals. So yeah all other exp from other content is just wasted unless you SB or level your chocobo and even then, a merit system or conversion to gil would be great especially for people with all level 50s and Rogue still not in until about 4 months from now, let alone other classes people want to play or for people who just don't want to level certain classes not related to their mains which is also acceptable.