Annihilation's best usage right now is to pressure targets that aren't being focused by your team's main assist train. A good example of this is locking down a healer (or two) while your team bursts down an enemy damage-dealer.
You're going to contribute less if you try to use Annihilation to focus fire targets when you have, for example, 2 or 3 Pyro PT's on your team that kill targets faster than can you get your peak damage set up. If you want to assist in focus fire, both Carnage and Rage are more optimal in that regard.