2) It would add a lot of overhead to the at least the server if not everyone's clients to do such calculations. Can you imagine how many mobs get killed all the time in an MMO...
3) Having to partition XP like that in a group would be a unpleasant on resources on a large scale.
4) What about healers? Athene has a pally.
5) Unforseen problems since you can bet your ass Blizzard would have considered such a thing. Etc.
People who are at the cutting edge, will always find a way to seemingly game the system. If it wasn't this way, it would be some other way. Blizzard just have to find a way to limit it to the areas they are least uncomfortable with. After 6 years, I assume they have.
Look at it this way, at least people like Athene are not cluttering up your NPC questgivers or slowing down your entire realm or instance servers running huge number of fast instances or even likely touching a single quest mob that you care about. They likely have NO impact on you as a player whatsoever!
Also, repetitive grinding has got to be the most tedious thing you can ever do. Most people cannot stomach it for even a few minutes. He did it for over 5 hours. It takes "special" people (and I don't meant that in a particularly nice way, lol) to do what they do.
All that will happen is within a few hours or days if Blizzard think they can quickly change or fix some of the mobs Athene and others used to grind out levels fast, then they'll just release a hotfix just like they've always done. Overall though, Blizzard do NOT design their game around grinders like Athene so their is no pointing even taking what they do seriously. It's just for fun.