The problem Dick is that all sorts of student pilots turn up with "certificates" that state they have Cambridge level "X" English. These are handed out no matter how poorly they perform once they pay their money. Its a scam and its going on in Australia and overseas as we speak.
I have first hand experience of students who are qualified on paper but are just effing useless. They included alleged graduate engineers and a Chinese girl whose life dream was to become a journalist for Rolling Stone.
The problem is compounded by the Asian tendency to say "Yes" rather than "No".
I saw Four C172's at Flight Safety once, all with the same damage (firewall) and all from the same cause (speed) - direct results of saying "yes" when the student hadn't a clue.
Hence the heavy handed approach of testing everyone. It is also not unknown for licences to be purchased and possession of one does not guarantee good English.
To avoid charges of racism, all must be tested.