The way threat reduction works is that fireballs get 20 threat redux on the initial fireball, then 20 off of each tick. AM gets 20 taken off per bolt.
Now lets make it over time and see how it works:
Fireball, 40 redux per 3 seconds, assuming you refresh your fireball DoT
Frostbolt, 100 redux per 2.5 seconds
Scorch, 100 redux per 1.5 seconds
AM, 100 redux per 5 seconds
Fireball = 13.3 redux per second
Frostbolt = 40 redux per second
Scorch = 66.7 redux per second
AM = 20 redux per second, however, AM doesnt need it as much because of Arcane Subtlety.
Scorch plays an important part in raid DPS - to steal and augment ignites, while keeping up the scorch debuff. Furthermore, all mages on aggro intensive fights that are fire speced use scorch, and this would give on top of that a nice, solid 66.7 redux per second.
Best for scorching mages, 2nd for frost, 3rd for fireball mages, because arcane mages don't need any redux help
