Well, in my case it wasn't, clever guy. The CM immigration officer neglected to stamp the additional 9 days I'd paid 1900 baht for. I even had the receipt at BKK airport. I then went to immigration in CM where their attitude was the same as yours. Initially, at least. They then soon changed their tune when they realised it was, in fact, their error.
If you never intend to return to Thailand then that's ok. Otherwise I would do all I could to avoid having an overstay stamp in your pp. I have one and it wasn't even my fault!