One of the reasons too is to see if anyone experiences that IO ask for onward tickets more on 30 days on arrival then on SETV. I want to book my home ticket to EU while I am in Thailand, 50-60 days. Also one reason I then can't get the SETV.
I have many times before gone on 39 days on arrival without any questions about onward ticket, bit I don't know the usual practices from the IO at the moment.
The company I fly with I know they don't check it!