From the Class & Balance Q&A today:
Q: Currently, Shadow Priests are very RNG dependent for Shadow Orbs. This can affect our dps a lot, especially the beginning rotation when we start dotting up the boss. It usually isn't as much of an issue mid-fight, when we can manage to keep the dots running with ES. Is there going to be any changes to the Orb mechanic in 5.0?
A: Shadow Orbs are random because otherwise you would always have a full stack when your Mind Blast was off cooldown, and you wouldn't even care about the mechanic. To address that in 5.0, you will never "cap" on Shadow Orbs. Mind Blast will cost 1 Orb, and have no cooldown. So the more orbs you generate "through RNG" the better, whereas currently if you generate more orbs before Mind Blast is off cooldown its kind of a waste.