As a user story, what we've actually done to preserve the XP that is destroyed when a personal quest is completed is have all but one human drop out, complete the quest with all NPCs attached which will reward all attached NPC characters (non-scaling), then have everyone log back in. Obviously this is ridiculous.
I don't think having the XP system tied to the personal quests is doing anything constructive. If there should be reward for completing a solo type quest, it should come in the form of an item that can be chosen to be shared or not. XP can't be chosen to be "need or greed"
