Wait really? So if the tank is taking less damage, thats a bad thing?

If you really want to nitpick about that, then why don't we add in healer damage if they don't have to heal the tank as much? I think that's more than the 3 potency a second dot we're talking about here.

50 additional potency a minute with contagion, with the fact that you also lose 10 initial potency from ruin, comes out to a 0.7 pps a second gain. I mean, I guess. You're right at least that its a slight gain over not doing it.