Drain Force and Stun

As others have said, you're right that it doesn't accomplish a great deal draining a stunned opponent in terms of delaying their cooldown. The real perk in your scenario, is that annihilate cooldown would be reduced by 5.

I used to run Palp under a Nihilus lead what seemed like forever ago, and I experimented saving "Drain Force" for the turn after stun. Ultimately, it made better sense getting that annihilate going sooner than later. Hope that helps.