I'm a regular SW flyer, and I often book one-ways because it gives more flexibility to cancel & rebook one leg of a flight if a lower rate comes out on Ding. Another reason to book one-way is when the first leg of my flight is included on their schedule, but my return flight is not. I have a flight for early June, but my return flight is past their current schedule, so I booked one-way for the first, and I'm waiting to book the second.
Hope that makes sense. SW is great!