I can understand why they don't want every BiS item to be buyable but I really don't see the harm in combinations.

BiS body and head and legs and weapons bought via new tomes, BiS accessories dropped by bosses, belt and boots and gloves craftable via super rare mats and master recipes but need to be penta melded to really be BiS.

That way you ensure that players are really doing all sorts of content and you're including both crafting and raiding at the same time.