I use a machine gun on my Hunter and a rocket launcher on my Titan(s) they're both useful in certain situations. I like the machine gun for taking down shields, the advantage being that you only have to use a few rounds then switch back to your primary and finish it off. A rocket is great but I feel it's a waste of a rocket to use it in that situation. Rockets are great for boss fights, machine guns are decent if they're long range but you have to stand there and shoot which causes you to be exposed to heavy fire but with a rocket you can dive in and out of cover shooting off rockets which keeps you alive longer.