As people said, it's just how you play it whether by subbing dnc or whm or use meds, buy RR scrolls or etc etc.
I hated doing campaign early on playing as rdm because all I got was a measly 500 exp from just healing or nuking. It just didn't seem to be worth it at all because for the same 30min in a campaign fight, my bf's WAR got like...2000exp. Yea, it just wasn't fair if you go and play as your assumed pt role. Curing, hasting, even raising random people did not make it worth my time.
It wasn't until I actually saw mages meleeing mobs and stuff was when I tried it out myself.
The difference was indeed like night and day. I cranked out 1500exp or so (maybe more) in the same amount of time. I was so excited with that lol.
So yea, instead of doing my usual mage duties, I just meleed, healed myself if needed. On rdm, I subbed whm for RR and I did just fine until some smartass thought it was funny to slingshot a NM turtle to the group of weakened people.
Sure it's frustrating to see a PLD not tanking, or a BLM not stunning, or a WAR not voking but it's all fair game there. They all want exp, they'll get it one way or another. Even if it means not choosing to heal others or voke, stunning, etc.