I noticed all the pve guides put versa ahead of haste, I made a post to get clarification.
Is there a pvp nuance that puts haste above it?
Haste is the same in PvP and PvE.
That said, I don't see any reasoning or math behind why Vers is better. Assuming we agree that defenses are not really important in PvE, and we're not interested in the healing or dmg reduction aspects of Vers:
Haste means more runes, more auto attacks, and more KM procs, and gives 12% Haste per 1000 rating for Frost DKs. Vers is always 7.69% per 1000 rating. 12% more Haste means 12% more rune abilities, RP, autoattacks, and KM procs. Since almost all our damage comes from those sources, this means ~10.9% DPS increase (removing disease damage which is not affected) in a long pve fight with high uptime.
Some people say it's because of GCD locks but that's bogus imo because as soon as you get 3 runes recharging Haste is fully working, even if you haven't emptied all your runes.
I don't see how Vers can even approach it.
There's also the fact that Haste is the Frost DK stat, and gets big bonuses from the spec itself.
Check out the latest results: http://www.simulatio.../Raid_T17M.html
The DPS 2h Frost DK gear is MS > Haste > Crit > Vers > Mastery, which is close to what you get if you use my numbers but remove the Crit/MS nerfs.