Though i dont play mage anymore and hasnt since 60, winter's chill is a must in a frost build imho. But maybe thats just me, as said it was a while since i raided with mage and lots have changed aparently with mage dps according to old guildies, but i clearly remember doing rather good dps as full frost back then, cant see why that should have changed all that much now. Also, as fights gets longer frost "improves" since its really hard to manage mana in longer fights where you might have to use healthpots instead of mana pots sometimes and the managems dont last all that long
/kasuki