I agree with you, well, yes and no...
I have a degree in Teaching English as a Second Language and I have been teaching here in Canada for over 10 years. I taught primary, secondary and now, I teach adults. I have seen just about everything!
Anyway...
I think that knowledge is one thing but in teaching, personality is also important. I am very dynamic, outgoing, patient, I have a certain talent to explain things clearly, etc... But I doubt I'd be able to be as good as I am now if I hadn't done my BA (that's what we call it here in Canada...). I learned the mechanics of the language, I learned how a child learns, how a teenager learns, I learned how to prepare good tests and exercises, things I would have never known otherwise.
French is my mother tongue and I am also bery good in French but I would NEVER be able to teach it, even as a second language! Well, maybe to beginners, but not to advanced students!
Just my 2 cents...
Sophie.