Every raid boss design out there have casts come up too soon, or cast too fast to rely on a paladin tank to interrupt. Until 4.0.6, neither one of our CD's are off the GCD. Meaning if you use an special move as part of your normal rotation and the boss decides it is a great time to cast, you are screwed. Also, hoping that you get an avengers shield proc to interrupt the next cast is a horrible idea. Rotating interrupts with a dps can work, but even then your interrupts might not come back up soon enough.