Yeah you could add in PS to lose on DS Healing + Life Shield. It wouldn't be that many anyway since every other Unholy Rune would have to be for DnD (with 15 sec CD).
I'd like to point out too that in Wotlk beta they kept changing dmg output from tanking abilities without also adjust threat modifiers at the same time so you would lose threat along with the dmg. They later raised the base threat of the ability so even if they end up lowering dmg again now I highly doubt theyre going to let the threat be lowered as well, at least not intentionally.