I am very much in favour of common politeness. Also there is the plain fact that there is no rule saying every utterance must have correct grammar and usage.

I am very much in favour of common politeness. Also there is the plain fact that there is no rule saying every utterance must have correct grammar and usage. But style guides that fail to describe the rules that have emerged over the centuries, and which actively avoid giving guidance on edge cases, do not help anyone. Especially not foreigners wishing to learn English.