Mana shmana. that's not the issue here. I barely run out of steam on my toons, if you spec into some mana friendly talents (and you should pick up a couple) you can burn through 10-20 mobs depending on the class you choose before you even think about stopping. Even my fire mage, who is mana wasting as hell, solo can burn 10-15 mobs easily between mana breaks. Granted, he's in T6 and has crit and damage out the whazoo. Running around with 5 toons, you kill everything in 2 button presses, wasting the second half because mobs die too quick, but you also loose a ton of time getting to mobs, looting, and I'd say in most areas where mob population is kinda spread out like normal - 20 yards apart, you'd only see marginal gains with 5 toons compared to 2.

Where 5 really shines is how quickly it gets quests done. Later levels, it's ALL about questing. The rewards are huge, but it takes longer for 1 to kill 12 rhinos or whatever, and a 5 boxer can burn through it fast and be on to the next one. Dualboxing is still gonna be faster than one, almost guarenteed, but it's not too burdensome to take on collection quests and such, which can lead to big xp gains through chains. I'm currently dualing Warrior/priest, and just hit 77. Now that I got my flyer back, I'm plowing through quests, because being on a flyer changes everything. No more getting knocked off while crossing the zone, and the ability to drop right on top of a quest mob and killing it without killing every mob up to him - priceless.

Leveling fast = questing fast.