Here's a theory: As NSX pointed out in another thread, you can't cancel only part of a Basic ticket. So if you book a roundtrip and the price of one leg has gone down and the price of the other leg has gone up, you can't automatically take advantage of the lower price on the one leg. Although there's an easy workaround (don't book Basic tickets as roundtrips), I suspect that the vast majority of WN bookings are made as roundtrips and that people will reprice less frequently than they used to.

Of course, this is giving WN some credit for thinking this through. Maybe too much credit . . . .