Frostfire Bolt, why?Follow

I have been wondering this for a while now and would really like to know why people use Frostfire Bolt.

As Frost, you get survivability unmatched and Frostbolt does -tons- more than Frostfire and slows more.

As Arcane, you get incredible DPS. Since update, I don't see any other spec matching the 57/3/11 output.

I use Frostfire Bolt when I am soloing Frost and come across Ice mobs but aside that, the slow cast, low damage, and pitiful DoT makes it seem worthless.
Though I figure with proper talents it has to do something.

But, what? I'd really like to know why it's used. :)

It is good for raiding. It has a very large crit multiplier and gets the hit talents from frost. When you have better gear most people switch to arcane/fire or just arcane. FFB is an easy spec for lower-geared mages at 80 in raids.

You won't get a 40k crit w/o some heavy out of the ordinary buffs.

To be a little more specific, for a frostfire bolt spec you go deep fire with some in frost, normally a 0/53/18 spec.
This gets frostfire bolt all of the fire crit and dmg additions/multipliers, plus the iceshards talent for another 100% crit, icy veins, frost channeling for reduced mana cost, piercing ice for 6% more spell dmg, and precision for the 3% hit rating.
Your non crits with ffb will be lower than fireball(in fireball spec), crits will be higher than fireball, and your dps will be of a lower consistency. Sometimes insanely good, sometimes blah, depending on your actual critical rate fight to fight.