The pre-upgrade rank 12/13 items are so poorly/cheaply itemized that for almost every single class they are replaced by BWL/AQ items. For some classes(hunter, rogue) they are even worse than items that can be obtained in MC. For some classes even the upgraded sets are replaced in BWL. As far as pre-upgrade sets the only items that are considered "good" are the ret paladin and warrior sets.
Playing 16 hours per day for 9 weeks(r13, 1000 hours) for items that are barely better than tier 1 loot from Molten Core seems absurd. The PvP sets should be upgraded with the launch of ZG like they were originally on Nostalrius. Many classes already start replacing items from the upgraded rank 13 sets in AQ40, so why make everyone wait until they are obsolete in Naxxramas?
Without this change I don't feel like many players will even bother ranking on the fresh realm as the rewards are completely worthless. I mean, what premade will have a hunter ranking if he knows that the items he will get are MUCH worse than 8/8 Giantstalker? Also he will know it will be 18+ months until he can even use the items he received from his insane 1000+ hour grind. The same can be said for many classes.
Nothing untoward came of releasing them with ZG on Nostalrius so I don't really understand the change for the fresh realm.