It is VERY easy to show just how much damage you actually loose.
Let's take this build, which is very close to BiS http://ffxiv.ariyala.com/10N4F
We have 1238 crit, 700 ss (enough for one more Ruin3 in Trance), 228 Piety
And let's meld 110 Piety into it. Since crit is better than ss, I will first replace ss. I got something like that: http://ffxiv.ariyala.com/10N4O
We got 1238 crit, 590 ss (we LOOSE one extra Ruin 3 in trance), 337 Piety.
1 Piety=8 mana. We got 109*11=1199 extra mana at strat. 1 extra Ruin3 out of trance.
Mana regenerates at the rate of 2% each server tick. It means 109 Piety lets us regenerate 1199*0.02*20=480 mana per minute. So we get 1 extra Ruin3 each 2 minutes. BUT we loose 1 Ruin3 every minute because we don't have enough ss to cast one more Ruin3 in trance.
So if we take a 10 minutes fight we loose 10-5+1=4 Ruins3 (we loose 10 Ruins out of trance and get 5 Ruins due to MP regen and 1 Ruin3 in the beginning).
And I didn't even count in loss of potency of our DoTs due to lower SS.