You could argue that an affliction warlock is like a shadow priest with a demon. Or that a destruction 'lock is like a fire mage with a demon. But it's hard to argue demonology as anything but warlock, especially when a demo 'lock is often a demon with a demon.
And yet for a long time, demonology had been the lesser of the three specs. In Wrath, it was popular opinion that demo 'locks were only there for the raid buff, despite the fact that well-played demo could be competitive and even beat the then-reigning affliction spec. The other popular use of demo was to farm either old dungeon fun or honorable kills as the nigh-unkillable, hybrid affliction, SL/SL-specced drain tank.
In Cataclysm, demo took a change for the better in its DPS output. It rose every tier and finally overtook affliction and destruction in Dragon Soul. But its ascent wasn't -- and still isn't -- without problems.