I've been asked and it was the one time I had a one way flight into Thailand with no planned exit AND no valid visa. Coincidence, perhaps?
I had a special letter with me from Thailand Elite explaining in English that I will be met at the arrival gate and taken to immigration where a 5 year visa would be inserted into my passport and I would be admitted into the country with a 1 year stamp.
At the end of the day that's all they care about - that they don't have to transport you back without a booking as it becomes an issue for them.