The BoT has tightened up on non resident accounts as an anti money laundering policy. that is why it is so hard to open an account now, and presumably SCB sees requesting a TIN as the best way of ascertaining residency. There has been something like this with non resident accounts in the UK with suspected non resident accounts being frozen, possibly in accordance with the same international agreements. SO, it is not impossible that you may have a situation in the future where your Thai bank account is frozen until you produce a TIN.
I am just looking at possible scenarios and knock on effects.