I dropped from 5 boxxing to 2 boxxing mid-WOTLK (after a 6 month wow break), find it more fun. I dual box all the dailies, zone quests, etc so I only have to rep up once. I single box heroics (twice a day basically), I run a dk/mage so its insta-Q as tank which is nice. Having fun with wow that way, plus I raid with a guild. Still, after a month of cata getting kinda bored and looking at Rift in the betas.

imo 5 boxing PVE is kinda dead for most people, sure you can wait 6 months until you overgear everything but that seems the equivalent of using cheat codes in a game, whats the point. LFD really killed it for me, before that it was sort of hard getting groups together for heroic dungeons so 5 boxing made a lot of sense, and for the holiday events it was nice soloing stuff so you could give nice drops to "your main". LFD makes finding a group a breeze, and they changed the holiday stuff so everything is bop.

It'll be interesting to see how PVP goes, rated BGs seem kinda lackluster/dead compared to arenas still, but if Blizzard tweaks the rewards to heavily favor rated BGs I think you'll see arenas also slowly die off. No clue if multiboxers have a easier/worse time in rated BGs.