It seems to me that alot of these things (specs) are chosen because some sponsored individual of a guild who's sole purpose in life is to mage away.
I think few people take into account reality. The fact of the matter is, aside from some fights in which one spec is clearly favored over the other, both specs are quite similar in terms of dps and damage done; sometimes lacking in one area and making up for it in another.
Unless the fight is ridiculously short, typically, unless the consistent dps rate far surpasses the capabilities of the other regardless of anything you can do, arcane will be the better dps and superior for burst; whereas fire will be the better in damage done. This certainly isn't an always case scenario, but it's something you could likely expect.
Take into account raid composition, player competence, reaction, latency, gear. All these factor into what you're capable of ultimately as well.
Sure, rawr can help you ballpark yourself, but the majority do not consistently reach those rawr parse numbers.
All the specs are closer to another in terms of dps v.s. damage done than ever before, and now is likely the best time to chose whatever fits your play style.
Would a LK encounter be easier with an arcane mage? most likely, would it be doable with a frost mage present?, most likely.
Don't be FotM, use what you want, and make the best of it. Some outstanding mages can blow away the newest cookie cutter 'must-be' build that elitist jerks says rawr says you should be.
Not every story you hear of a fire or frost mage being neck and neck is an instance of a baddie arcane mage (because lets face it, it's not very hard to arcane mage).