You cannot bring too much firepower to a gunfight. However, unless the fight is choreographed by a Hollywood producer you can only fire one weapon at a time. Therefore, to carry on the fight if the weapon you are firing such as a shotgun fails for whatever reason, jams, or runs out of ammunition you need to transition to another firearm, a handgun for instance.
If your shotgun fails and you are within handgun range then it is better to sling your shotgun and engage the enemy with your pistol. This may give you time to fix the jam or to begin a combat reload. You cannot allow the aggressor to advance on your position as you fiddle with a jammed weapon. You need a sling to maintain control of the shotgun. If you do not have a sling, you have drop the weapon to engage properly with your handgun. You really do not want to give up control of the shotgun if it can be helped.