Your secondary is the weapon you use to get back to your primary. (usually a pistol)
Oh S&%T my M4 is out of ammo, transition to your (glock, sig, .45, etc) to stay in the fight until it's safe to get back to your primary. Carrying 2 long guns is dumb. Carrying a long gun and an SMG is dumb. Pick up a pistol AFTER you've picked up: Camo, boots, glasses, extra batteries, low/mid cap mags. Your secondary is an 'Oh fuck" weapon when your primary goes down.