Fates are spammed because SE didn't really provide any other means to get EXP on your other classes. Dungeons are okay-ish, but since the majority plays DPS the queues are horrendous unless you're tanking or healing. The non-story sidequests are okay, but they're not really great on their own and dry up eventually. Battle Leves? These things are worthless, they give a paltry amount of exp, which is ridiculous since you dont get that many allowances.

The only *REAL* source of consistent EXP in the game for your other classes, comes from fates. SE needs to take a look at stuff like making Battle Leves actually give a meaningful amount of exp, possibly buffing dungeon EXP, and allowing Guildhests to be done at full EXP on other classes besides the first (though limiting the gil to the first time clear is probably a good idea). Until SE makes changes to the other leveling methods, people are going to continue to spam FATES.

Nerfing FATES without buffing the other methods would not be advisable.