Foe is really useful in dungeon when the tanks is taking mob and there is a BLM.Best example can be seen at Brayflox i think.My blm(i90 almost full myth +leviathan staff) pulled 2 flare at 2k8 damage on multiples mobs.Actually if there is no need of Mp song ,foe is the best if you got a BLM/SMN.We did 4min52 on a speed run at brayflox and this number could have been improved if a BLM with more gear had my place(BLM is not my main class,i don't buy anything for him)
Foe is also great during raids IF you know you're not going to need the MP song soon.We use it at T6,if there is no unecessary damages,it really up the dps as hell.
Peon is pretty much useless i think if a melee or a BRD is having problem with his PT,he's having way too much SS or he doing stupid rotations.

In final i would say:Priority to MP SONG ,only FOE if you know you're not going to need the MP for ballad soon.