It's odd that whereas e-gates are used to enter and exit in Malaysia and Singapore, as of a few days ago, Suvarnabhumi only allows e-gates for exiting, and as you mentioned, you would receive a stamp upon reentering. However, it appears that you can enter through e-gates if you have a passport from Hong Kong or Singapore. Other passports will eventually be able to enter through e-gates, I suppose. Additionally, I made the error of not informing the officer that I had an LTR the time before I entered Thailand, which resulted in a two-month validity stamp! 🙄