This is the subject of debate in every ESL teacher forum I have ever visited. Logic would state that, yes, when it comes to teaching something, those who are experts are better qualified, and who could be better qualified to teach English than those who have learned it as their native tongue? But the answer is not that simple. Many native speakers have no idea how to teach although most of them think they are good at it - teaching that is. Even some expats with University degrees and teaching certificates are poor instructors.
On the other hand I have never met a Vietnamese English teacher who could speak English properly (sometimes not at all) but almost every Vietnamese English teacher had superior grammar knowledge.
It seems quite simple to me; every student should have some instruction from a native English speaker to improve their pronunciation.