Answer a simply question, if you are facing Assault grenadier charging with sprint would rather have 2 Bren on your IS or 1 Vicers K and 1 Bren?
Rather simple, indeed. A pair of bren will start to deal dmg from afar (may be event before the sprint if you can see fist) and have chance to drop 1 to 2 models as assault gren close the gap. One the gap is closed, with either combo you will have to retreat or at least move anyway so why bother a more expensive weapon which cant win you the contact. Do notice that while being chased by cqc unit, if you run backward (common reaction), you often cant fire back much.