The AOE damage is just not enough. The flares don't last long enough, are too small, and don't do enough damage to justify using an exotic slot. Yes, a stationary boss will get hit by the flare damage, but its weak. The initial blast makes up the bulk of the damage, you are better off using a legendary RL to free up the exotic slot, if you have Ghorn, then there's no reason not to use that. It could have been, (and could still be if bungie patches it) a decent exotic, but as of now, the flare is just too weak.