Textbooks are an important resource for teachers, but they are just a resource.
Since textbook authors are typically not aware of specific needs in a school's curriculum, it doesn't make sense to base mathematics instruction entirely on the recommended scope and sequence of a particular textbook.
The nature of a textbook suggests that only a very limited number of examples can be included, which leaves the classroom teacher as the primary resource for meaningful.
This is an important point when considering that only the teacher, not the textbook, is capable of assessing and addressing the needs of the individuals in the class.
However, a good textbook allows a teacher to base lessons on interesting and relevant problems without having to spend too much time inventing or searching them. In order for the teacher to be successful with the textbook, he/she needs a deep mathematical background as well as a clear vision of the scope and sequence not of the text, but of the grade level being taught.