I think haste is more useful for a frost mage actually. You FB casting cost is lower to start with so more over a shorter period of time means more damage at the cost of mana, it moves you closer to fire in damage and mana usage.
It is often hard for a frost mage to totally go through all their mana in a fight... this moves you closer to being able to do this and moves you up the damage meter.. a fire mage using the same strategy would also move up the meter, but may find themselves manaless before the fight ends.
Frost mages are going to use freeze effects more... and of course... if your able to freeze stuff immobile with your frost bolts a certain X% of time you will freeze it more frequently over the course of a fight if you are throwing out more bolts over the time of the fight.
Also you have your cooldowns on some effects that can freeze/slow critters. If you are using cone of cold to slow groups as you blast them, you can get off more shots with spells that are hasted before and bettween cone of colds.
Don't forget haste also affects channeled spells now... so two blizzards off during a timespan when you could only get one blizzard off before is HUGE in a giant AV battle. Any situation where your lifespan is limited by time either from huge amounts of incomming damage effects or because enemy's are advancing behind the group you are attacking, or adds are being summoned into fight.. etc.. haste makes you more effective overall. Especially if its a suicide run already....
Lastly... hasted spells are harder to counter as well as harder to interupt. I think it has massive benefits outside of the simple increase in DPS.