Balance becomes an issue with this concept since then you run into the problem that content will be to easy since large groups can take down open world foes and they all can obtain rewards from it. They wouldn't really be acting for the sake of teamwork but it would turn into the same issue like the old "skillup" lv system that 14 had. They would hold the mob to cap out their potential reward and then kill it.

Open world claiming will/should fix itself come 2.0 in which it should take on a more FFXI concept.