Well, don't forget that SMN can attack multiple mobs at once. Even without using Bane, just putting DOTs up on multiple mobs is ridiculous for Potency/MP efficiency.
So in instances where you might have to trade Fester for Energy Drain, you can also have multiple DoTs running on multiple mobs. So while you may lose 12.5 DPS by using Energy Drain half the time over Fester, you are also doing X * #Mobs extra DPS vs a single target rotation.
It takes 11 Casts of Ruin (869 MP) to equal putting your 3 DOTs on one extra mob (890 Potency @ 398 MP) that's about a 70 DPS boost vs single target in just 3 GCD (on just one extra mob)
So, if there was an 11 minute fight without any other mobs, but the boss, I could see where SMN would fall behind.