I've leveled every class to 50 (I'm working on both tanks atm, both at lv 40ish)....

while leveling up every class, the dungeons might be always the same, but your playstile change completely. every time you do a dungeon, with different class/different roles, it's different every time. you get the chance to learn how to play your new job/class, you see the dungeon with a completely different perspective...

I'm more bored of running the same dungeon every time with the same class (hello, Quarn Normal, I'm looking at you), but then again, you still have to adapt every run you do, to the other party members. it's not always the same "spam spells on mobs while you look at your tumblr".
Light farming Garuda HM/Ifrit HM is waaaaay worse than this xD