The issue of credentials in language teaching and language skills is one that I find a bit of a conundrum. Just as a high TOEFL score is not a reliable indication of what a person can do in English, a TESOL degree does not equate to a good teacher.
If I ran a school, and certainly for our tutors here at The Linguist, I tend to put more trust in a university degree in the Humanities. At least the person has read widely and had to defend his/her opinions in writing. (But even there I have seen some recent graduates whose use of English left a lot to be desired). More than that I would like to read some things written by candidates or have conversations with them to see how well they can express themselves.
There are many short term TESOL courses offered which more or less award certificates for attending the course. I have often found ESL teachers with credentials to be very conservative in their approach to teaching and unwilling to look at more innovative and effective ways to teach. They seem to cling to their credentials and the “skills” they have been taught at school. People without that training are often more open minded and more effective communicators, motivators and therefore teachers. But that is just my own experience.
The core competence for an English teacher is the ability to use the language well. Are credentials more effective in screening out bad teachers than the self-interest of the school to hire good teachers? I do not know.