Fire mages have much better mobility than arcane mages do. In these cata boss fights, you are constantly moving in every fight. Arcane mages can't move or their stacks will run out of time and their dps will be screwed. Fire mages can cast every spell except fireball while moving, thus making them much better in about every fight in cata.
In a lot of fights there are also AoE phases. Arcane has absolutely no AoE whatsoever. Fire mages have one of the greatest AoE's in the game currently, and arcane mages don't compare to them.
Fire mages aren't even that far behind arcane mages for single target fights. If a fire mage really knows how to play, really knows how to get good combustions, there is not much of a dps difference between a fire mage and arcane mage on non-movement fights.
A lot of you think arcane is going to be so much greater because the whole spec is getting a 13% buff. Since arcane is a one spell rotations and arcane blast damage is getting increased by 13%, that's how much more damage arcane mages will be doing.
If you know anything about fire, you will know that fixing our ignite bug, will also increase our dps a good amount. For 1, we will always get good combustions if all of or dots are up. Since our ignite will only be from direct hit crits, the ignite tick will be much higher, making our combustion higher every time. It will also increase the total damage of ignite because instead of ticking for 3k from a dot, it will tick for 15k from a pyroblast crit.
Fire will still be the better spec as it is right now.
It has much better mobility, not too far behind single target dps(non-movement) and is much, much better for AoE than arcane.