Quote:
backstabs aren't mitigated much
Backstab is treated like any other melee attack. Your attack rating vs. mob ac, take your (base damage + damage modifier) + (damage range - mitigation) = your damage.
The only reason it might be thought of as being mitigated less is it is not subject to parry, riposte, and dodge if used from the rear of the mob.
I don't know if there are specific calc's used for large HP mobs, but I can say from my experience that my Rogue will backstab for more DpS vs. a raid mob than a non-raid mob. (Basically more higher damage range of hits).
As far as DpS of a Rogue, their attack rating is capped at the same point as a warrior, where as a ranger can go higher, making them push out more DpS in general. Backstabs are definately more efficient and less aggro producing than a wizard nuke, etc.
Just my opinion on it, but a rogue will pump out alot more damage during a longer fight than a caster, say averaging about 180dps from backstabs, whereas a caster
can do more damage, but the aggro generated would lead them to a short end, or at least a long med break before they can continue to add DpS, not to mention resists, again putting melee in the forefront.
As for multi-boxing, caster's are much easier since you can fire and forget using macros for casting at the push of a button and med while you manage your melee toons, and as you said mob positioning isn't an issue really. Personally I avoid using DD caster's in my multi-box kit because they die way too fast. Drink lots of coffee, and get a swivel chair or an integrated keyboard network for all 6 comps, and use paint-markers to highlight your important macro / hotkeys for fast recognition on your keyboard, and have a ball!