Arcane Spec provide the highest DPS burst for single target and it can also be very efficient with mana for low downtime, playing arcane spec is also simple and easy to play for beginners as an alternative to frost (not everyone like to play with combat pets). However by having the highest single target DPS spec, Arcane suffers by not having a good Area of Effect (AOE) damage for solo grinding.
Frost Spec is also a great leveling spec that is completely different play style than other specs. The Water Elemental is now a permanent pet and you can also re-summon every 3 mins if it dies, the Water Elemental can also absorb a lot of damage for you and the Permafrost talent will help a lot with keeping it alive. Combining Blizzard with Ice Shards is also an easy way to grind multiple mobs giving Frost one of the best AOE spell for leveling.