I think that a lot depends on the shotgun being used as well as the type of shot, for enstance I think a sawed off or coach gun would be near useless except as a final defensive secondary, but soemthing like a Remington pump or Benelli M4 would be usefull in many situations, lets not forget the fact that most shotguns can also shot non lethal ammo as well, which would be usefull with dealing with people that are militant, but don't warent lethal force.
if a living person wants to kill me they are going to have to deal with more than a rubber bullet, what I ment by not warrenting lethal force is maybe someone trying to steal your stuff... they are no threat to you but you want to keep your stuff, and who knows they might be usefull as an ally... and if they aren't, that is why they next round is a frag 12.
While you bring up good points, at some point you will need to expand the size of your group and create a community. To some extent that will mean letting in strangers, but of course you take their weapons until you feel you can trust them not to kill you. If you don't let them in, then there will be little chance for hummanity to repopulate.
I didn't mean it like that!!!! Of course, you are going to need a little help, so make friends! Just don't trully trust them until you really get to know them.
A benelli m4, although it is an excellent weapon, can not use non lethal ammo. Because it is semi-automatic it can only use low-powered rounds. If you want non-lethal rounds use a pump action shotgun or a benelli m3(semi auto/pump action). I agree with you on the fact that you should not use lethal shot against humans. This is because if you start by killing enemies soon you might start killing everyone who has what you want. Even when zombies come you have to stay human.