you can just look at the how it went with horde v alliance pre tranquil air. There were still horde that progress very rapidly. Now there is a tranquil air totem on horde side which will make learning the instance easier(to some much easier). I dont think blizzard will ever make a set bonus imperative to learning an instance.
Only two mages in the guild bothered to complete the Arcanist set, and I think it only really helps in the Broodlord fight -- elsewhere it's absolutely unnecessary, and you only gimp your damage output by wearing it. And even in the Broodlord fight, you end up spamming Arcane Missiles(with 3/3 in Subtlety) out of habit and a desire not to screw up a kill and force another 20 minute re-clear. And really, if that's all you end up doing, you don't really need to wear 8/8 Arcanist anyway, since the 40% threat reduction is enough (and with an additional 20% from Tranquil Air nowadays, the only way a mage will pull aggro is if the healers suck and let all the tanks die).
False. I've written up a couple long summaries about the arcanist set, because I used the 8 piece for a long time. If you respec subtlety than there's no need at all and you gimp your damage potential by a lot as falc said already.