Actually, my understanding is that you do have to have DING downloaded to get the airfare. To get a great fare like the $29 offered last night for one-way travel between IAD and MCO (going both ways) you must book online, but if you just go through the SWA website you won't see the DING fare. If you are aware of the DING fare but wind up calling SWA reservations to try to get it, they will tack on a service fee because you called rather than booking online. So, the bottom line is if you want to get the DING fare you need to download the program.
If I am mistaken about this, I'm sure someone will correct me.