Just found this article -
http://www.wowwiki.com/Death_Knight_(hero_class)
If you scroll down to the lore section it reads -
"The death knights of the Second War were powerful undead necromancers created by Gul'dan to replace the warlocks slaughtered by Orgrim Doomhammer."
It's all beginning to make perfect sense now. Warlocks are suck in PvP to fit in with the lore of deathknights. At least we all now know why warlock survivability in PvP is zero. :'(