Now if you want to say you think the merc as a whole is short on AoE that is at least a valid argument (even though I don't agree). The heat cost is certainly high but along with thermal sensor override it does give pretty decent bang for the buck.
You can't be serious, can you? Explosive Dart costs 16 Heat, which takes 3.2 seconds of recovery time to dissipate. In exchange it does AoE damage to up to 3 enemies. Compare that to Smash. Smash costs 3 rage which takes 2.75 seconds of recovery time (Assault) to generate. In exchange it does AoE damage to up to 5 enemies. And the damage is about 3-4x greater than Explosive Dart. In what imaginary world would the Merc's AoE be considered a "decent bang for the buck"?