Shared XP (and frankly quests except your starting one) by default is also a HUGE +1 for me.
If I am playing Co-op I am playing it, unthinkable as it may be, for CO-OP...
The whole point of the "Competitive Questing" is that you do not have to come to the same conclusion. Not so you can be directly antagonistic towards eachother.
Actually diminishing the exp gains of your allies directly harms you.
