People would abuse this. A tank can pull a mob and just sit there forever while the healer spams heals. In 8 man parties, the two tanks would be trying to MT simply because the OT would get way less EXP. Healers would try to heal only MT for more EXP. The idea sounds nice on paper, but that's about it. Incentive to get better scores would only cause people to play badly in a different way for the sake of more rewards.