I always hear from people that sooner or later I'm going to have to respec Fire or Arcane for tougher raids. I know the limitations of Frost, and it annoys me (each tree should be able to competently raid anything, yet still have its defining strengths and weaknesses), and I'm only going to respec if it's absolutely necessary (...I like doing more damage than rogues ><; ), but that's not what this topic is about.
When I ask senior mages which spec is best for raids, I almost always hear "Arcane". And I always hear the same reason for it: "Two pieces of T5".
The 2-piece set bonus from T5 just makes Arcane Blast 20% stronger. You can't spam Arcane Blast forever because of its horrible mana inefficiency, but even if you could, a 20% damage boost to one spell is not enough, IMO, to justify respeccing your toon entirely; you can't base your entire playstyle around one spell.
Not just that, I've heard from some that the T5-Arcane spec combo isn't good anymore for some reason. Usually those people suggest Fire or Arc/Fire.
So I'm going to ask the mages here. Which do you think is the better tree for raiding between the two, why, and what are the strengths and weaknesses of both trees for raiding?