Simply put: Ratings were implemented in WoW, because in vanilla, items just said adds 2% to crit. When the level cap was bumped up to 70, they had to implement ratings or else low level items would still have a use at 70. A rating system right off the bat indicates that the D3 devs know the level cap will be increased in an expansion and they are just preparing accordingly so they don't have to shift the system later.