Hybrid exp penalties do not effect the entire group. They don't give cooties, contrary to (popular?) belief.
When a mob dies, the first thing that is calculated is how much exp everyone gets. Using the delicious pie example, 100% of the pie is distributed based upon the levels of everyone in the group. Once this is determined,
then the exp penalties get applied, per individual.
So everyone gets their slice of the pie. Then right before each person eats their slice, the penalties hit and anyone with an exp penalty gets a piece of their pie cut off before consumption. If it isn't working like this, then it's a bug.
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Also the ability to tank != the efficiency of tanking. Plate is meant to give a constant mitigation that gives healers more time to react; since the incoming damage is constant and not bursty, it is much easier for a healer to say "At 30%, I'm going to heal" and know the tank is probably going to make it until then, whereas a Monk is subject to failing a couple avoidance checks and taking more damage more unpredictably.