This was a buff to the solo warlock PvP or PvEer, as they only ever had their own affliction debuffs anyways, which allows their own Drains to be more powerful when playing alone.

It is a balance between the solo warlock and the warlock's synergies with other warlocks. Besides, it was a class specific synergy (warlock synergy with warlock) and does not promote gameplay using a variety of classes.