Give me a fight where you wouldn't want a Blood DK...don't worry, I'll help you out. Garajal HC, Sha HC, Horridon HC, Ra-Den, Malk HC.
Now give me a fight where you'd love to have a Blood DK.
I think what the blue said about DK's has been true this entire expansion somewhat (can't fully speak for both DPS specs), aside from Unholy having a moment in the sun with Festerblight in 5.2.
As a tank, I think that statement is true as well.People see patterns where they aren't. Nothing to do with scaling as you imagine. DKs simply lacking fights to shine this tier.
But that's just my opinion, (I play both a Blood DK and a BRM monk)