When I was a kid, studying at a school in England, we had a French teacher from Senegal. Her English was pretty poor. As a class we cut her to pieces and made it a nightmare.
In my experience those who say we shouldn't speak Thai in the classroom aren't teaching 40-50 kids once per week who understand very little English. Likewise, those who argue that speaking and understanding Thai is essential probably aren't teaching advanced students on a well run English programme.
Having taught in both contexts (as well as at schools in between the two), context and appropriate judgement is key. However, it's important not to be completely reliant on using Thai to solve every issue as there are more effective strategies you can use in the classroom