From my viewpoint, on balance, TEFL courses are useful but there are a number of drawbacks to the courses as well. I have never personally done any of the courses (But did do a one weeker at AUA before starting my first teaching job) and rarely teach English these days, so I am more or less an "objective" observer" and have no interest in defending my own decision to take a course or not to. One of the problems in these courses are inherent in all short training programs which can be seen in business, the military, and just about all types of occupations, short courses focus on practices instead of the principles behind the practices. This sometimes results in individuals using the practices in the wrong times in respond to the wrong situations. Because they lack the theoretical underpinnings, they may not understand the contextual aspects of what makes the practices work. Another problem is these courses have a tendency to instill a "there is only one way to teach" mentality. This is especially problematic with head teachers and the like, who may believe the way they were taught in their CELTA or other courses in the "right way" and any teacher doing something different is "wrong." I am not even close to an expert, but I am familiar enough with research done on education and epistemology to realize there is no clear agreement in academia about the "best" way to teach or learn. The CELTA (or others) way may be an effective way to teach English, but no one should think it is the best or the only way to teach. So I think the attitude of the student makes a world of difference. If a student expects to pick up a few tricks of the trade, get a chance to do a bit of practice teaching, and gain a foundation to continue to learn on, the course will do some good. On the other hand, if the student expects to become an expert teacher who now knows how to teach and believes all people without the CELTA or similar course shouldn't be allowed to clean toilets in a school, it will do more harm than good.
Then there is the economics of these courses. The CELTA at ECC runs $1,400, and with lost wages for the month, one can expect to forgo 80,000 to 90, 000 Baht. How long will it take to pay off the course? If one is just doing a one year teaching lark before beginning one's real life, will the cert give you an extra 7 to 8 thousand extra baht a month to pay for it? If one plans on making English teaching a "career" than obviously one needs one of the certificates, but it seems like a lot of money to pay to get "qualified" for a very low paying job.
Finally, we must give a lot of credit to the TEFL training industry in selling the notion that a four week cert is needed to teach English. When I first came to Thailand around 8 years ago, the TEFL training industry was just in its infancy. The vast majority of teachers had no training at all and only a very few people made a living from providing TEFL training in the country. Today, many (Majority?) of ESL teachers have some form of cert, and a number of people have created a whole new industry and are making their living from TEFL training. There are no four week courses for teaching economics, physics, math, natural sciences, biology, political science, Japanese, Latin, ancient Greek, European history, or the hundreds and hundreds of other subjects taught around the world every day to billions of people, why only English? Actually, the selling of the TEFL training industry might make a good research topic for an academic marketing paper. Also the emotions these courses provoke in there defenders and detractors is puzzling.