Many jobs I read on ajarn.com request native speakers, i.e. people from English speaking countries. However, I have lived in the UK for the last 25 years and have good command of the English language, originally from Holland. I have studied in the UK, been to college, polytechnic, and I worked there. Since 1987 I taught at a further education college, though not English, but computer literacy. What I am trying to point out, you can have people from a native speaking country and from a region thereof with a very strong dialect.
In England, take a Cockney, someone from Yorkshire or Newcastle. No criticism against people from those areas. But people in other countries will find it hard to understand them, or ask someone from Glasgow or Northern Ireland to teach here if they speak with their local accents. The students will be totally lost. When I lived in England (before moving to Scotland), I knew many people from those areas, and with time you learn slowly to understand a little what is being said. When I lived in Scotland, I had plenty of friends who originally came from Glasgow.
Personally I love the English language, it has got a rich source of literature. I think that people from non English speaking countries who have spent such a long time in an English speaking country are very capable of making themselves understood. Of course, one must not speak in the local accent of the area where one has lived. I lived in Birmingham, England, and started to pick up the local accent. No, it is not a rant and rave, or even a moan, just a general statement, and trying to find out what is really a native speaker.