George Chilton has an excellent post (with some equally excellent feedback) that can be found here. A key criticism of the use of coursebooks is that they are designed to compensate for the fact that many ESL teachers are undertrained and underqualified, and are not capable of designing their own curriculum and preparing the relevant materials. I agree with this point, as when I first began teaching I had no training whatsoever in TESOL and had absolutely no idea where to begin. My first project was in English for special purposes, and there was no textbook. I had a tendency to teach too much too quickly, and expect content to be absorbed without giving the students enough practice. I also had a heavy grammar focus,which was completely inappropriate for students on a short course in need of functional English for their work.
So would I have been more comfortable and taught more satisfactorily should I have had an appropriate text to help me along the way? Most definitely! I think it would have allowed me to create a plan for the course as a whole, the points that we would focus on and examples and practice that I could provide to students.