The threat reduce is minimal, and unless your tank is within a few % of a dps pulling it off, it wont cause any threat problems, but the damage reduce of it is godly on enrages and heavy hits like frozen blows6. I very rarely use pain suppression and never on the MT since it reduces threat. The only time I use it is if a dps has pulled aggro.
Dont forget you also have hymm of hope for mana regen8. I very rarely run out of mana and when I do, I use shadowfiend first as in long fights it may be usable again before the end of the combat, then I'll pot if shadowfiend is on CD and I still need mana. Usually shadowfiend will come back from cooldown before the mana from the pot is gone.