I never said that you did, but if you want to nitpick...
"@medium gear level the difference isn't that big as well due to frost needing arpen."
This statement is completely wrong.
The only reason Unholy gets close to Frost on Single Target DPS is one and only Shadowmourne.
Without SM, Frost is a LOT better.
Also, the only reason Frost isnt topping other single target DPS specs is since it doesnt have a huge burst CD like Death Wish, or Wings. Give a good FDK Hysteria and you will see what happens...
As for tanks, part from Bear beeing the better main tank, and Paladin better off tank, 90% of Blood dk tanks go for some magalaga spec like:
https://wotlk.evowow.com/?talent#j0EMqIhIcbobsoxhgZ0gh
(Or even worse, like going for "Might of Mograine")
So yes, Frost dk dps is needed in raids for both Raid buff and its DPS.
P.S. A good FDK will always top Shadowrealm Halion, and be high on LK and Ragings DMG on LoD, so idk what you are talking about...