Smooth bores sure can hang with rifled barrels, but I agree most do not.
You can shoot rifled slugs in a rifled barrel, they work great. My gun shots both sabots and rifled slugs to the same point of aim at 50 yards and there is not difference in group size at that range. I use them all the time because they cost less and hammer deer. The new rifled slugs travel 1800 fps, are full bore, weigh more, and expand on impact. The rifled slugs are rifled in a different direction then your barrel so they always fill the lands and grooves for good rotation. You find they work better in a rifled barrel.
The sabot issue is not so clear. Some can be fired through smooth bores and where designed with that in mind. They are hour glass shaped and the front in heavier then the back so the fly much the same as a dart does. You will still double to triple the size of your groups though when they are fire through a smooth bore vs rifled barrel. I've seen some shoot under 5" at 100 yards though. Copper solids and the newer pistol bullet style slugs should not be fired in a smooth bore. The copper solids should never be fired in a choked gun.
A smooth bore gives you the option to use the gun for shot, buck shot, and slugs.