I tote around a G36 with five 50-rounders, and a KSC USP .45 with three twenty-rounders. I really only use the pistol if I run out of ammo with the G36 and I don't have time to reload, or if the battery dies in a similar situation. Though we, as a group, usually like to close out our games with some CQB pistol-games, so that makes it worth it. Depending on where you're playing, you probably won't want to be hauling a rifle in your hands, a machinegun on one shoulder, a shotgun on the other, and an MP5 strapped to each leg anyway. Less is more; just carry spare mags and batteries.
It's all fun and games until someone gets hurt...and then it becomes sport!