Using a heavy machine gun is more of a "high risk, high reward" type of thing. It doesn't kill instantly like rocket launchers but potentially gets you more kills than rocket launchers. Typically you get 4-5 kills from a rocket launcher if you are good. But 7-8 kills with a machine gun if you are good. That's potentially 16 kills you get from heavy compared to 10. But again, rocket launchers are more reliable at getting kills. I use machine guns. [spoiler]The better question is why use Gjallarhorn in the crucible. PvE guys are really annoying with their stupidity.[/spoiler]