Exo's only value is to be a backup tank and Mana Seal. I originally levelled Mana Burn for PvE but i'm finding myself not even bothering with that. Once i get a better tank like LK or Spartan, i'm going to remove Mana Burn completely and max Mana Seal for use in FL and that's it.
Even now i'm finding that the Exo's value is to Mana Seal staff types even in PvE. Mana Burn is weak as hell and for some reason, Mana Burn + dual Mana Drain (exo+witch combo) still cannot suck a monster line totally dry, therefore it's pointless.