the value of a native-speaker as a teacher
A native-speaker is the best choice as a teacher. Of course, there are various quality levels with native speakers... some NS s (native speakers) are uneducated, speak difficult dialects or use too much slang, etc., but this is a subject for another entry.
I can illustrate the value of a NS this way.
Have you ever used a copying machine? Have you ever noticed how the copy of the original is slightly worse than the original. Now copy the copy. What does that do? Yes, the copy of the copy is even worse.
Now apply this analogy to the teacher whose native language is not the target language he/she is teaching. He will be teaching his "version" of English to his students!
The results will be a further deterioration in the students speech.
This explains why many students of English living in other countries have such difficulties in listening comprehension and pronunciation.
John