Something I never considered was the possibility of having Grenade Launchers in the Kinetic and Energy slots. If they keep their current ammo counts and damage profiles and numbers (particularly in PvP) they could work in the K/E slots. I also want to sit in a Luna-rift and Rambo-hip-fire a Grenade Launcher while watching chunks of the health bar disappear from a Boss. Tough call.
They need both in PvE. They do less damage and have less splash than rocket launchers so crowd control is limited, and since they do less damage they require more shots to kill which leaves them have using up more ammo for the same general damage.