Having EXP scale with contribution will just drive more people to not do UQs, and simply do more raw grinding in expeditions/etc. Solo.
Final Fantasy XIV has a minimum contribution floor that guarantees a certain amount of EXP depending on how much damage is dealt to a standard enemy along with enemy to player level difference. However, if someone tags the enemy with an auto attack or attacks it first in any way, that player gets full/maximum contribution of EXP. This is to prevent griefing of EXP gains where high level players destroy low level enemies even though low level players attacked them first. I believe a couple of Aeria Games' MMORPGs don't have this failsafe, so some high level players will just run around killing enemies low level players are killing, and those low level players won't get any EXP.
When it comes to HUNTS in FFXIV however, enemy aggro is one of the factors that determines contribution.