Single barrels can be fun and quite effective. However, assuming we're talking game birds here, there are some species that are more likely than others to require a 2nd hit to bring them down, or to anchor them if they hit the ground running and you're dogless. For that reason, for example, I would not hunt pheasants with a single barrel gun. I'm not a waterfowler, but I'd think waterfowl in general might be another category where a 2nd (or more) shot would come in quite handy.