At BKK (esp. for domestic flights) it does work pretty much like that: there's always a mob of people milling around trying to ferret out the best price if they buy a ticket right now from Air Asia/Nok/12Go/Thai/whoever. Remember, most LCCs also guarantee that their highest fare is still less than the full Y care on the full service carriers, so unless the FSC happens to be running a promo that applies to last-minute seats (which is unlikely) you will still save money... albeit not as much as by booking in advance.