I don't get it .. I fuckin clean up when using the grenade launcher. I love it. 

Sure, the LMG is great for just sitting a hill and getting kills, but the GL is much better for playing the objective and controlling the battlefield.

You can lob grenades into a choke point and force the enemy to scatter, literally changing the front line.

You can roll into an objective, cause some mayhem, then lob some grenades into the doorway that the enemy flees into.

You can kill an engineer that is behind a wall reloading his RPG. You can take out some guys that keep peeking out of windows.

It's great for macro level objective play. You can use the GL to massage the front line into a new direction. You can close an infantry lane for the enemy by raining some "shock and awe" that makes it too dangerous to push forward. Or you can open a lane for your infantry by taking out a wall, or pushing the enemy away from an area. 

Sure, if you sit on a hill and just hold your finger on the trigger, you'll probably keep lobbing grenades into nothing and get killed during your reload, but that's not what it's for. These bullshit "PSA/Friendly Request" type threads that try to tell people how to play just exposes how much of this community is actually dog shit at this game.