arcane is a better spec than fire or frost imo, especially for multiboxers. fire beats out arcane by a slight margin when you have best in slot (naxx25/maly25) gear and are doing 25 man raids only and have 320-360 hit, which is not something a multiboxer should be aiming for. While its true you lose the 10% extra crit from scorch being on bosses, the fact that elemental oath is up so much makes it not that big a deal, plus 10% crit is not that big a damage boost as you may think. You can spec frost of course as well, its just arcane is the highest DPS spec with varying levels of gear (requires the least amount of hit).

as arcane you can make a castseq of

#showtooltip Arcane Blast
/stopmacro [channeling]
... put target stuff here, or use [target=focustarget,harm][] in next line...
/castsequence Arcane Blast,Arcane Blast,Arcane Blast,Arcane Missiles,Arcane Barrage
/use 13
/use 14

(you can drop it to 2 Arcane Blasts and remove the Arcane Barrage if you want). Sometimes I do spec my mage fire for raids and when multiboxing I don't bother with pyro at all, just something like

#showtooltip
/assist focus
/castsequence reset=combat Scorch,Scorch,Living Bomb,Fireball,Fireball,Fireball,Fireball,Fireball, Fireball,Fireball,Fireball,Fireball,Fireball
/use 13
/use 14

obviously this is a loss in DPS due to no pyros or extra living bombs but for heroics and VOA (places I multibox) I find it no problem at all.