All of my kits have dual roles.
Like the OP, my Assault kit uses the rifle as the primary weapon and a shotgun for close quarters. He also has C4. When not breaching, he's part of the maneuver element (with the rifleman) to assault enemy positions.
The Marksman is also the medic and ammo-bearer (one additional LMG magazine and one additional rocket).
The Rifleman, like the Marksman, is also a medic and ammo-bearer.
The Grenedier (Rocket) uses a rifle with the rocket in the pack, while the LMG has a pistol as a secondary firearm.
I rely heavily on LMG for suppression to enable my riflemen and assault to maneuver near the enemy to destroy them in close combat. Frag and smoke grenades are used heavily. C4 is used for breaching doors and walls, and for reducing obstacles. Ditto for rockets, especially when armored enemies are concentrated in a small area.
As everyone has noted, maneuvering the LMG and rocket into a firing position is difficult, and needs to be timed well. When moving into the enemy's field of fire and unable to fire that turn, smoke combined with the use of cover is key to increasing the odds of survival. It helps having a medic immediately on hand. In some circumstances, either weapon can be fired after moving around a corner if your assault element is close enough to the enemy to destroy them by the end of the turn.
Gaining superiority of fires (i.e. destroying or suppressing the enemy) is absolutely essential. Both LMGs and rockets can accomplish this or enable it if used correctly.