I don't buy into the weapon's retention argument. I don't think that anybody of average ability and training should worry about weapon's retnetion. As I've said before, few burgulars are burly experts at homeowner disarmament. The pistol grip is an effective tool in VERY tight quarters to keep the muzzle pointed at the threat. In true retentioni terms, the butt of a standard stock may be used as a lever to disarm a homeowner.
That having been said, while few are trained well enough in weapon's retention, still fewer are trained in proper use of pistol-gripped shotguns. Besidse, if you want to engage people anywhere but the confines of a closet, you might want the stock. A case in point is the submachine gun. Notice how the BATF always shoulders their weapons? Heck, it's a full auto. The reason that they don't fire it from the hip is simply that this is not the most effective way to utilize a weapon of any sort. Put it on your shoulder and shoot it.
As to pistol-gripped stocks, I think they are fine for the majority of situations. They are easier to control in some situations and many feel more comfortable shooting them as the middle finger isn't scrunched against the trigger guard when firing from the hip.