Dark Intent is not a essential raid buff any more then Focus Magic is, Bloodlust and such buff a entier group while Dark Intent buff one person. They needed to lower the damage of Mind Blast as well which would not have been accomplished with enrfing DI, altho i do agree they could have lowerd Mind Blast base damage and then removed the dot aspect of DI from the person which is it put on. That would not nerf locks and it would bring shadow priests in line while not making them as dependant on locks.