TL;DR: I make the obvious case that 4-piece + Uvanimor reduces the value of Haste so much that it alters the BiS list, including which tier pieces to skip.
To start, here's my DK: http://us.battle.net/wow/en/characte...igtyr/advanced
On Tuesday I got the Unholy 4-piece, and was wearing Draugr. After my current belt dropped yesterday, I swapped Uvanimor in as my second legendary because Unholy 4-piece significantly reduces the number of Festering Strikes you use in a fight and so Draugr value plummets.
I simmed myself before and after the legendary swap.
Before swap: Draugr + 885 Gul'dan ring, with 32088 Str, 15.17% Crit, 18.21% Haste, and 86.71% Mast
531761 DPS; Str = 13.49, Mast = 12.75, Crit = 11.51, Haste = 10.7, Vers = 10.55
After swap: Uvanimor + lucky belt, with 31317 Str, 16.19% Crit, 21.62% Haste, and 83.77% Mast
533930 DPS; Str = 13.77, Mast = 12.84, Crit = 11.37, Vers = 10.59, Haste = 10.47
Keeping in mind that even with the Draugr setup, I felt like I had too much Haste. Drowning in resources. Swapping to the Uvanimor setup threw an additional truckload of Haste onto the pile, bringing me to that magical 21% threshold, but sinking its value to dead last.
Clearly the value of Haste would diminish once you get to 21% Haste, but if you were lucky with gear like I was, 4-piece + Uvanimor (and even Draugr) already makes Haste pretty bad. I think I had somewhere around 17% haste after picking up the 4-piece initially (Gloves instead of Chest) and I was still GCD locked and overcapping both runes and RP.
But ultimately, having 4-piece + Uvanimor makes Haste so bad that it alters the BiS list from what is usually considered, i.e. which tier pieces to skip.
In particular, I find that the tier legs and tier hands are my BiS simply due to the fact that Haste is worth so little and is overcome by the sheer strength on those pieces compared to the lower ilvl stuff from Chronomatic Anomaly. Wearing those two "non-traditional" tier pieces means that I want the Crit/Mastery shoulders from Botanist and the chest from Gul'dan (due to it's Strength). Due to the haste on tier legs and Gul'dan chest, I think it's unlikely that these gear changes will move Haste up on the list.
Once T-19 4-piece goes away, Draugr value will return, and once T-19 goes away entirely, I'm sure the value of Haste will go back up to where we expect it to be (i.e. you want to hang around 21% or so). But if you're like me and got lucky by getting Draugr + Uvanimor, it looks like 4-piece would let us hang around 17% Haste or even lower without adverse consequences.
I said a lot of stuff that's probably obvious, but I wanted to get some additional thoughts and discussion on this.