Our mages need to have a fire spec for heroic ICC 25 modes.
First, fire is able to do more overall and consistent dps than arcane in almost 90% of the fights in ICC. Arcane is useful for extremely high single target burst damage and slow, but rarely do they compare to living bomb and hot streak procs on multiple targets.
On certain fights, like Valithria, fire falls behind, mainly because each add isn't alive for the complete duration of living bomb, which makes the encounter much more suitable for arcane's high single target damage.
So whether you want to keep arcane as a secondary spec with a few pieces of gear to swap out for the hit difference, is up to you. It may also be useful to have an arcane spec for 10 mans where slow is more important in some encounters, but you need to have a fire spec.
You can armory me and copy my spec if you wish or you can make your own. A good fire raiding guide can be found:
http://www.empoweredfire.com/2009/10/05 ... guide-3-3/You can also read my second post, which will go into detail on how to create a good fire build.