frost pvp crit ?

I'm an inexperienced pvper, so can someone please explain why most frost mages only run with 17%ish crit? Yeah I know mastery stacking is very powerful for frost, but aren't mages completely reliant on crits for damage? Should I only get to 17% or so, even if I'm not planning running with a rogue or feral and start stacking mastery after that point?
Thanks, 00troll
I run with 15% lol.

It's annoying to have my damage be RNG but I would much rather have the huge stupid crits than low crits that don't do that much damage.

It's not that big of a deal, just frustration.
Mastery is more RNG what the !@#$ burst, where as crit is more sustained, as much as you can be with a frost mage lol.

Honestly I dont think it makes that significant of an impact.
Mastery is guaranteed to be used, where as crit isn't. That is, when you don't crit all the crit rating you stacked got wasted. Mastery however, is always used.

How many times do I have to link my friends calc?

http://ragial.com/mdamage/

Please read his thread that is linked to within in the calc.
On my main, i run with 18% with complete reforge to mastery, 3500 resil with the heroic tier leg.
Even at 15%.. you get 5% from Glyphed Molten Armor. Having a Fire Mage or Destro Warlock puts 5% debuff on the target, and a few classes (and hunter pets) provide 5% crit buff. This is 30% right there. 30% x 3 = 90%.

Let an arcane mage FM you, and you are at 33% = 99%.

EVEN at 15% and nothing else available, that's still 45% chance in reality. So it's really not THAT low. I think even in PVP it's important that somehow you get close to 20% (with buffs). 60% just puts the odds in your favor.

EDIT DEPT: Didn't see the "pvp" aspect in the title my bad, lol. But like I said, even without any other buffs/debuffs, that 15% is considered 45% crit, which is still pretty good.