This blog actually started with my wanting to expand arguments I had made several places - most notably at EJ. Seems appropriate that a post I make there spawns my first post on DK tanking here.
Caveat: I don't know anything about DKs, really, other than they're the tank that looks like they belong in a rave when using their abilities. However, one thing I do know a lot about is parry mechanics. Here's the post I made over at EJ:
How we figure out whether a DK is better off using a 2h or 2 1-h comes down to a few things. These would be nice to have, data wise, but don't end up mattering hugely.
What is the typical expertise value of a DK?
What is their typical avoidance %?
What is their typical HP?
They don't matter because either the DK is dealing with content that is trivial (in which case min-maxxing tanking weapons is not important) or the DK is dealing with content that is not (in which case their avoidance, expertise and HP are on the cusp of what is needed to deal with a boss).
After that, we need to take typical tanking weapons into consideration. I would say that a 1.6 speed 1h would be typical (I realize there is a great, slow 1h in naxx, but that is not the standard), and a 3.0 speed 2h weapon would be typical.
The special hit rate between 2h and 1h will remain unmodified. That leaves only white attacks and weapon speed to consider.
With 2 1.6 weapons attacking, you will have 3.75 as many attacks in 3.0 seconds as you would a 2h weapon. This means you have 3.75 times as likely a chance of being parried. Clearly, as you get slower tanking weapons this value goes down. The question is whether the value of using two tanking weapons is worth getting parried almost 4 times as often.
Comparison time. Let's take [Titansteel Destroyer] vs two [Red Sword of Courage].
The gain of 76 defense rating (15.4 defense, or 1.83% total avoidance) is great, but the loss of 74 strength is pretty poor. Still, that's only 18.5 parry rating, or a bit less than .4% parry. So the total avoidance gain is around 1.5%. So to deal with being parried 3.75 times more often in terms of white attacks, you are gaining 1.5% avoidance and about 12 stamina.
To me, this doesn't sound remotely good enough. And note that using the comparison above, it's actually worse; you get parried 4.25 times more often from white attacks.
Finally, note that the overall parries is not 3.75 times as many times due to the special attacks also having parry chances. In a 200 second fight, you would have 133 special attacks (assuming one every gcd, which I admit is fudging things), 67 attacks with a 2h, and 250 attacks with 2 1h. Or 183 more attacks total. That is 1.9 times more than with a 2h alone. So basically you're trading at best 2% avoidance for a doubling of the chance to be parried.
Really, not worth it to me.
Now, what I didn't say at EJ: the effects of a boss parrying you are hard to model, but fairly easy to understand.
When a parry occurs, the target that parried the attack has a chance of having the current swing hasted, as follows:
* If the next attack would normally occur within 20% of your weapon speed after the parry, there is no effect.
* If the next attack would normally occur between 20% and 60% of your weapon speed later, it happens 20% of your weapon speed later instead.
* If the next attack would normally occur more than 60% of your weapon speed later, the time until your next attack is reduced by 40% of your weapon speed.
The overall effect of a parried attack is to speed up a boss's swing timer by roughly 30%. In the worst case it is 40%, but that doesn't happen all the time. ) This means that for using 2 1-handed weapons, you are giving the boss 30% more haste. You have effectively bloodlusted the boss after each parry. 30% more normal swings may not be a lot for someone like Rage Winterchill, but for Patchwerk? And remember, this is not 30% more swings over a nice, long extended time; this is 30% overall more damage that comes in tasty, bursty chunks of death that tends to take your healers off-guard.
Note that I'm not saying doubling the parries makes the boss have bloodlust the entire fight; it depends a lot on how much expertise you have, and I've written about this and parry-gibbing prior to this.
Now, if you're expertise-capped for parry that's something else entirely. Then you'll be trading basically some damage for some avoidance, and that's a great thing to trade. But getting to that 64 expertise value (or 500 expertise rating) is going to be pretty tough, and I bet that you could have just gemmed/enchanted for avoidance and skipped the expertise part anyway.
ETA: one thing I did not factor into consideration is the ability to use two enchants. Having dual goose would help somewhat with the mitigation factor, but even then we're talking about at best 3% total avoidance vs. 30% boss haste. That still doesn't sound like a good deal to me.