PvP stat requirements makes PvE gear useless, and PvE do PvP one.
They lost one of components in WoD so removed second: substitution took place
not because MoP system didn't work, but because they decided to remove rating PvE characteristics, which upset overall balance between gear from different activities.
But they don't want to bring first ones back, that's why they turn up their nose now out of second - this is something stupidly simple, which still most people in such topics don't want to understand. And this is after fact that whole system in
MoP looked quite acceptable for majority due to this reason, but which, unknowingly, people continue to attribute to class design at that time (it wasn't bad, especially after what happened later, but main reason is balanced set of characteristics).