I tentatively disagree

Once you have verified that your script runs with taint-checking, then taint-checking serves no further purpose

To correct your analogy, taint-checking is the irritating voice that tells you when your seat-belt is undone - once you fasten your seat-belt, this voice should not utter another word, and therefore what would be the difference if you turned it off entirely?

I would agree that it would be possible to write a script that would be vulnerable if taint-checking was disabled, but that would imply a faulty script and/or testing process (e.g. you never bothered to test the script using all available params)

Once more with feeling - taint-checking doesn't make your data 'safe' - it just prevents your script from processing unsafe data. The mechanisms you place in your script to make data 'safe' will still be present irrespective of whether taint-checking is on or off