I think that Deathknights should beable to tame scourge, I mean there are plenty of types of undead.
Banshee -> Utility caster
Shade -> Utility melee
Lich ->Utility caster
Vaklyr -> Utility healer/mage.
Spiked Ghoul -> Upgraded Ghoul, like Raptor-> Devilsaur conversion
Lesser Spiders ->Utility melee
Skeleton (Armored/ not) -> melee
Gargoyle ->Utiltiy caster
Precious (dead hound) ->Utility tank
Just change Summon Gargoyle into a "Tame Exotic Scourge",allowing them to tame some of the cooler undead like Valkyr, or a Precious like dead hound, it'll give death knights some diversification (and maybe unholy a shot at PVP?)