Here's my general observation from the SWA side of things.

More often than not a one-way ticket purchase will "random" a pax. I've found this to be true for non-rev listings as well. Over the last few years I have noticed that when we list non-rev, one-way on SWA we are always a "selectee". If we make our listing roundtrip (even if we have no intention of using the return) we are never selected.