I shall start this post about native teachers by asking you a simple question: "Do you think you could teach your native languae, say Portuguese for foreigners?", or " Would you know where to start or even what is relevant, in your own language?", then "What's your experience in learning your language from the point of view of a foreigner?" Therefore, knowing whatever you want to teach is far from being enough. ; you MUST KNOW HOW to teach it; that's a KEY word when it comes to studying languages. Then, I have talked to native speakers whose English, in terms of structure and vocabulary was rather poor , basic mistakes as: "She don't know nothing about that". But then, people will argue by saying that well, at least, there's communication...Ok, I understand. Still, I'll play the devil's advocate here and ask them:" Is that the kind of professional you would have at your company? on the other hand we have foreigners who spend years studying the language,  methodologies, teaching tools, checking books, planning classes carefully so you can take full advantage of  your course.