Airlines in general have been doing a better job to match inventory (seats) to demand (passengers). As such, there are less empty seats and therefore less incentive for airlines to discount seats.
I believe it certainly pays to subscribe to Ding and if you fly Southwest often enough, to lock in on a low fare if you think you might make the trip. With Southwest, even if you cancel, the funds sit in your account for future travel for a year from the date of purchase. If you're not a frequent SWA flyer, this strategy might not play as well for you.
As you likely know, there's no cost to subscribe so why not do it? It's particularly useful if you have more than one airport that you can conveniently travel from so as to be able to shop prices from a few originating points.
Dan