With the obvious better option of using fragments or perks that grant volatile, echo of expulsion serves almost no purpose as a fragment. Also, it should be noted that besides the breeches, void has no fragments that grant class energy. Every other subclass has fragments that can grant grenade, melee, and class energy. I propose that echo of expulsion should grant some class energy for each enemy defeated by an ability in addition to its main effect (enemies explode on ability kill) to give it some usability.