It's always a good idea to go into combat underestimating the enemy, right?
So let's just assume the bad guys will always run away...
Realistically, they usually will. The question is, what happens when they don't?
How many shells does the 12ga in question hold? Does it have a spare ammo holder of some type on it? Have you practiced speed reloads with it? Reloads on the move, while taking cover?
Do you have a backup handgun to go with it, in case you run out of ammo, or don't have the shotgun handy when the intruders enter?
In my case, the answer would be five rounds, no spare holder but six spare shells near the gun, and I'm not as fast with the shotgun as I should be, so a multiple BG encounter could go south pretty quickly. I'd rather have my AR.