It certainly makes the teacher's job easier. Whether I use one or not depends on the course and what books are available. Many school districts have cut back on textbook purchases, hoping to save money, but many teachers wind up using photocopied material in the absence of textbooks anyway, which can cost even more. There are many disadvantages to textbooks, and teachers have to be willing to use supplementary documents in addition to the text, but for most high school courses, they are essential. What is important, then, is picking the best textbook available, which for me means one that most reflects the direction scholarship is going.