I'm not thinking "OMG, gotta prove that DW tanking is inferior!" I find the idea of emotional attachment to a particular way of holding weapons kind of silly. If tanking with a fishing pole were ideal, you'd find me out dutifully grinding Kalu'ak rep. But until it is superior, you're not going to see me zoning into raids ready to fish (unless we're slumming it in SSC or ZG, I guess).
We need to clearly quantify the advantages and disadvantages of dual-wield tanking.
The disadvantages and opportunity costs are clear: you will generate more parry-hasted incoming attacks until you are fully expertise capped, you need to be frost, you need to sink at least three talent points into propping it up, and you'll need two tanking weapons at least one of which is slow. (If you're about to reply to say either "Nuh uh!" or "Those don't matter!" stop and keep reading, because we've got about a page and a half of that already.)
The advantages are [?]. Please fill in this blank, with math preferably.
Nobody here (well, not me anyway) is yet saying "DW tanking isn't viable" or "DW tanking is just plain dumb." I am saying that DW tanking has a clearly identified cost in increased incoming damage, as well as a handful of other costs. What does that currency buy?
Also, parting shot:
Using my own math, we can show that DW tanking results in ~2.7 extra parried attacks per minute, assuming no expertise and a post-3.0.8 rotation. (That's the 18.2 extra parryable attacks multiplied by the 15% parry rate everyone figures bosses have.) The one extra attack per minute assumes a reasonable amount (I forget exactly how much) of expertise and the "Each parry is a 40% speed-up" number, and is Sartorri's estimate. It's a reasonable one, assuming my math is good and the tank has two slow weapons.Using his own math, we can show that DW tanking results in 1 extra parry per minute.