Randolph Quirk say: Non-native speakers are not suitable as English teachers because they do not have the basic knowledge in the phonetic rules of English, further more they lack the ability to speak the language fluently by heart so to say. They will never be able to speak without mistakes and don’t have the grammatical structure of the language as their back bone in their ability.
Braij Kachru says: Non-native speakers are better teachers because they can identify with the students. They know the process of learning a new language and can focus on points where they already know the will be problems. Non-native speakers have been taught the rules and are therefore aware of them, whereas native speakers use the rules without being aware that the rules exist.