Unholy is seen by many (including Blizzard) as being too strong PvP-wise on WotLK and is considered the only PvP-viable spec for DKs on WotLK exactly because it's the only one capable of dealing with healers in any way. Not that it has healing reduction, no, but it's still better than a button-mashing Retadin who secretly hopes that the healer forgets to look at his HP or goes AFK in arenas, or that the stars align and he 100-0s a healer in HoJ with all cooldowns.
Unholy has pressure which Frost and Blood don't have. Frost can be quite sticky with Permafrost, but one Abolish Disease or a talented Cleanse and you're ****ed. Blood can't stick to its target in any way possible, so it's a no-no. As Unholy, you're permanently slowing your opponent with Desecration in addition to already having Chains of Ice and DG. Your diseases are locked from being dispelled and you have your ghoul which constantly slows spellcasts and can stun. And just to finish it all up, there's your gargoyle which when used correctly can put enough pressure to either force a powerful cooldown or land a kill.
For PvE - Unholy is viable and sought after for any 25-man raid. There is no other class in the game, which can provide the magic damage increase on multiple targets. Your warlocks will also love you because they can use other curses for higher DPS.