I beg to differ. I've flown many times on Thai roundtrip for less than $50, sometimes for free, when booked in conjunction with my international inbound to BKK. Even if booking your domestic trip separately though, there are still some advantages to Thai. If you connecting from an international flight, the transit to a Thai domestic flight is quick and painless. With Air Asia, you must go through immigration, collect your bags, go upstairs and check in with Air Asia, then off to your gate. The cut-off time with Air Asia is 45 minutes and I've heard it's strictly enforced. It all means you have to allow a LOT of extra time if you're connecting on Air Asia vs. Thai. My time is very valuable to me. It isn't worth it to save a few dollars and have to wait around the airport for several extra hours. Of course it all depends on the timings of your inbound/outbound flights. In some cases it might not make any difference if there's no sooner Thai flights departing than the next Air Asia flight.

Another disadvantage of Air Asia is their miserly baggage allowance. If you're over, even by a bit, you'll have to pay the overweight charges. For people traveling international and connecting to domestic, they usually have a lot of baggage, so besides the hassle of collecting/rechecking their baggage, they also have to pay the extra charges.

Getting miles on Thai is another advantage, though at only 500 miles it's certainly not much to talk about, though still for a few might make a difference in reaching a status threshold.

Air Asia is also well known for scamming customers out of their money when something goes wrong. Your chances of getting a refund even if it's Air Asia's problem are not very good and if you do manage to get a refund it will likely require a lot of hassle and time from you.

If you're only flying domestic and want to save some money, Air Asia might be for you. If you're transiting though to/from an international flight, for me it absolutely isn't worth the extra risk and hassle.

I have a few Thai friends and have on occasion asked them to join me at some domestic location in Thailand and have paid for their plane tickets and they flew either Air Asia or Nok Air and that was fine. I didn't want to spend a lot for them to come join me and they nor I didn't matter if the plane was a few hours late or not. But for me personally I avoid them all together, and I'm not someone who normally spends money I don't have to.