Rifled barrels have recently come into popularity for use with shotgun slugs. The new sabot slugs require a rifled barrel to gain the optimum accuracy of which they are capable. The conventional forster slugs and the brennke slugs may also gain some accuracy when they are fired in a rifled barrel.
An option for those that don't want to spend the money for a rifled barrel is to get a rifled choke tube. These are extended chokes tubes that stick out from the muzzle a few inches, and have rifling engraved inside. Since the slug or sabot is already moving very quickly by the time it enters this rifled choke tube, it may skid on the rifling and not spin up to the full angular velocity that it would have gotten in a fully rifled barrel. However, these rifled choke tubes typically cost a few $10's rather than the several hundred that a rifled barrel will cost.
If your intent is home defense, rather than hunting, I recommend that you stick with a smooth bore barrel. At the distances inside a home, the increased accuracy of the rifled barrel is insignificant, and the rifling will cause any shot loads, including buckshot loads, to have a doughnut pattern, decreasing their effectiveness.