Well if you were to swap 31 Crit (2% Crit Chance) for 31 Pie (215 MP)
43 MP/Aetherflow + 86 MP/min (129 MP/min = 1.63 Ruins = 130 Potency/min if you are MP starved (~8 DPS @ 512 DPS)
1% Damage from the 2% Crit = ~5 DPS
But because that's only if you are MP starved and say 5 minutes into the fight.
that 1% Crit is 1536 Damage where as the MP = 0
Further down you might have 10 minutes where you are limping your MP and touching 0 before Aetherflow comes up.
Crit = 3072 Damage @ 10 mins ~ 5 DPS
Pie = from 5-10 minutes @ 130 Potency/min = 2600 Damage from extra Ruin casts. ~4 DPS
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If you can swap SSP for Pie then it's no question.
Edited: I would like to add that PIE can also reduce the number of Energy Drains you would use over Fester over the course of a long fight. So in that regard I suspect that PIE is pretty close to the weight of Crit (up to the point that it offers no additional benefit)
I.E. - 1290 MP over a 10 minute fight = ~ 5 Energy Drains worth of MP = ~ 5 DPS
Estimated 1/3 Energy Drain without Ballad over 10 mins
So you could probably stack 60ish PIE with the same weight as Crit. But too bad that it wont effect the pet damage because using i110 Crafted pieces with PIE would probably be more of a loss to pet damage due to primary stats from i130 gear.
I wouldn't even chose i125 Zodiac with PIE over i135 Dreadwyrm for that reason.