I think this area can be a real minefield. There are ESL teachers whose English is not great, but they take the jobs seriously and give proper, productive lessons. And of course there are plenty of lazy fools among native-speaker teachers. And then there are lazy fools among the non-native speakers....
I think when assessing the quality of a language teacher, there are many factors to be considered, and the quality of that teacher's command of the language is one of those factors.
My command of English is superb (I mean, look at my posts, baby! They rock! Well, maybe). But I honestly have to say that on average, my students probably give my lessons about 7 out of 10. I'm not fantastic at connecting in a real way with a large group of people - I kind of withdraw into the role of teacher. Stuff like that, which is hard to quantify, can be all part of the equation.