Re taking bird on the ground: Where I live, it is actually illegal to take sporting birds (pheasant, quail, etc.) on the ground. As well it should be.

Do shotguns tear up the fowl? For the most part, no. Once in a while, yes, but experienced gunners learn not to take those shots. Even on a shot that does tear up some meat, you can salvage 3/4 of the meat on a given bird with a pair of game shears and a filet knife.

Re using a rifle on flying birds: It can be done. I've seen a couple guys good enough that they could do it with some consistency if they wanted to. Could probably do it myself on some shots (a flaring bird in close isn't actually a very tough shot). Still, it is ill-advised and dangerous ... a lowly .22-caliber long-rifle shell can travel 1.5 miles in the air, and they gotta come down somewhere. Also, a warden catches you with rifle shells in the blind, you may have some big trouble, as waterfowl hunting is both state and federally regulated.

Re defense: I can see ironsraven's points about shells in the dark, storage etc. I never looked at it that way because I haven't had to. My sporting guns are locked away unloaded; my defense gun is not not locked away and is loaded (no kids at the house). I agree: I would not keep shotguns of two different gauges at hand for defensive purposes.

Sorry about all the "re"s and the definitive statements; i'm no expert; just trying to share my opinions on several topics quickly.