I have lvled frost and fire, and neither is faster if you know what you are doing.
Frost: AOE, maybe 5-6 enemies, use all mana, drink.
Fire: Solo, kill one enemy, then another, then another, till you get 5-6 (each kill takes about 10 seconds usually, where frost the mob takes about a minute.)
Then drink. It's all the same.
thrashering, as for your question, if you are speccing fire, then no, you really don't need frost bolt. however, it has its uses, not in leveling but in pvp sometimes. sometimes.
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OFFICIAL BLIZZARD DESCRIPTION OF THE DEATH KNIGHT
The Death Knight: A class that has complete control over the force known as death, they can manipulate it's powers to their every whim. At level X, they learn a spell called "Death," which is an instant cast spell with no cooldown that instantly drains all life force from a target, killing them. Also, since they can control death, they cannot be killed.