XP for grey quests is good because,
-as it is, a lot of people have to grind several levels for the lack of suitable quests. If you don`t want to grind mindlessly mobs you could always do the grey quests too
-some people enjoy the way to 80 more than beeing at level 80 itself. Those people tend to do all quests available.
-quests also tell a story and some even refer to other quests or are the beginning questchains which could probably go through all levles. If you never do the start you`ll never see the end and its rewards either. They impove the imersion of rpgs and usualy give a lot of background information. No XP means a lot of people missing content and ending in the US forums whining about the game lacking content...
just my two cents..