The specs some mages are posting are just horrifying, some skip out on permafrost and imp blizz all together!
http://db.vanillagaming.org/?talent#obhy0obZZVVGfRbtbo
You have a few points to play around with that spec (Perma frost and imp blizz or can leave in Imp CoC), if you want to just be annoying to anything melee, i would say 3/3 imp blizz is worth. But it's not limited to just melee, i often get many casters grouped up, watching everyone RP walk around while my team destroys them is a great feeling. It also has a chance to proc Frostbite.
bottom line, if you're going frost specced, in BGs your primary concern should be controlling and damage should come secondary.
If you're interested in dps primary and control second, i would suggest going elemental spec. You get the best of both worlds, ignite, frostbite, block, blastwave <3
Elemental spec has many different takes on it however, i prefer scorch specced ele, but again it's based on your preference!
Elemental is a lot harder to play than Frost, just because you don't have ice barrier, but with the dmg output you're capable of doing it shouldn't hurt that bad.