I agree with Mike that Sandstorm is not power effecient. I rarely cast it even in groups and here is why. If the fight is tough it means I have a lot of healing to do. In most cases I would prefer to have more power rather then provide a defensive benefit and then run oop. In a short and easy fight there is no danger to run oop but since the fight is easy no need to cast Sandstorm at all. The only real benefit (I feel) is provided in Sandstorm when fighting against a grouped target (3 or more). The increased defense gives more benefit to the tank (since several mobs have harder time to attack him) plus interrupts also give more benefit. I think it makes sense to use Sandstorm for a short period of time and then cancel it. For example, in raids cast it on the pull (tank usually gets more damage so it's more important to keep him up on the pull) and the cancel it after 5 seconds.
Leshii