As far as I know, the cited reason behind the ratings was to keep raiders from losing in the Arenas and still getting gear. The weapons in season 3 are terrific, even compared to the mid-endgame raiding gear, and so raiders were playing just their 10 games every week, saving up Arena points, and grabbing the hot weapons. Blizzard didn't want that happening, so they put the requirement on the weapons-- you have to be good to wield those now.
But the shoulders I don't quite understand. I've been told that the shoulders are the most obvious piece of "leet" armor, and that Blizzard, as Wreck says, really wants seeing those shoulders on someone to be a sign that they're really good in the Arenas. But shoulders? As a few players have said, give them a title or something if you want them to be recognized. There's good reason for putting the weapons behind a rating, but if you're going to put shoulders back there too, you might as well require the rating for all the pieces, or leave the armor out of it completely.