The ambition to do better than our neighbours is in many departments of life a virtue; in pronunciation it is a vice; there the only right ambition is to do as our neighbours.

Fowler goes on to acknowledge and discuss important subsidiary questions, such as who counts as "our neighbours" for this purpose. But the opening sentence, which I quote above, still tells you most of what you need to know.

Thanks to commenter wm13 who reminded me of the source, and of the precise quote.