You have 2 DPS though, and chances are if ones ranged and the other's melee, then you'll be giving both of them cards.
So even discounting both Tank and Healer completely...

With Div:
3 x 6% + 2 x 6%(from Div) + 3 x 8% = 54% over 3 minutes
No Div:
6 x 8% = 48% over 3 minutes

If you consider Tank and Healer combined to match up with one DPS (Tank damage is approx. 70% of a DPS, Healer is approx. 50%, but with more downtime in dungeons could drop it a bit more) then the Diviniation rotations goes up to 60% over 3 minutes.


Or another way to think about it...

Diviniation is a free additional buff on top of all of your other cards, and it equals 6%.

In order to use it, you need to reduce three of your 8% cards down to 6%. That's a loss of 6%.
So for one DPS, it's breaking even, and the AoE effect of Divination on your other DPS is a gain.