There is actually a surprising amount of research available on the internet about the pros and cons of course-books and how to select them. I personally do not use a course-book in my English language lessons, except in situations where I am helping a student to prepare for an examination, and then I would only really refer to the learning points required from the syllabus before coming up with my own lesson plans.
In my personal opinion, course-books can be restrictive and do not always account for individual student needs and they can sometimes take you down a path that is not totally suited to the needs of the class or student. The teacher / student relationship is in part dictated by the author(s) and publishing company when teachers are restricted to using only course-book materials 100% of the time, and I am sure that the influences some educational publishing companies have should be questioned when course-books are forced upon teachers.
There is a thread on the TeachingEnglish forum discussing this very subject, the consensus being that a course-book is essential, especially for group classes and to ensure all learning points required for examinations are met. However, it was also deemed necessary to supplement the course-books with additional materials from a variety of resources in order to keep students engaged and interested, which leads me to conclude that although a course-book could be considered a good source of lesson plans and relevant exercises, as well as useful grammar points, they are not the be-all and end-all that some would like us to believe.
I have also heard it said that only a lazy teacher relies solely upon a course-book without supplementing it with additional materials, but I don’t think this comment is entirely fair or should be used as such a generalisation, when I am sure that there are some teachers who do find their practices severely restrictive. Thankfully I have never been in that situation and am free to pick and choose my lesson plans and course materials as I see fit. This works for me, and I believe my students also benefit from not having to follow a restrictive curriculum when learning with me, it also has the added benefit of keeping things interesting both for me as well as for my students. A bit of variety is all you need to spice things up a little!
Another interesting point I have come across recently is one where both teacher and students get to choose their course-books together, based upon an evaluation of the relevant materials and courseware available to them. This is a fairly democratic approach which I am sure would work well in theory with classes of adult learners, although I doubt very much that you would be able to get a consensus out of a group of children.
There are undoubtedly advantages to using a course-book, they help teachers and school administrators to co-ordinate lessons, course-books aim towards a clearly defined set of achievements and objectives and they are generally consistent in their teaching of language skills. They are also extremely convenient, as everything is prepared in advance for student and teacher alike, but they are relatively expensive to purchase for individual students.
One of the reasons I am not a big fan of course-books, particularly in language lessons is the way they were used when I was at school. The language taught during exercises from course-books tends not to be realistic in my view. Eddie Izzard portrayed this point quite hilariously in his show ‘Dress To kill’ where he recalls learning in French lessons at school the phrase ‘le chat est sur la chaise’, which translated means ‘the cat is on the chair’. Not really the most useful phrase to learn in French for anybody, but it was typical of the language taught in French lessons at my school as I recall.
Living and working in Italy, I also want to give my students something different from what they are taught in English lessons at school. The Italian curriculum appears to be obsessed with English classics such as Dickens and the Bronte sisters, which in my opinion is giving school children the same experience from learning English that I had learning French. They are not learning useful, modern language which they can use in context everyday.
That said, I have found course-books to be a useful way to explain English grammar points in Italian to my students and they are useful to read again at home as sometimes they may not have fully understood, especially when they are beginners. So if they can read the explanations again, in their own time and in their own native language, it becomes an easier path to understanding. In class wherever possible, grammar explanation is done in English and I would rather my students took their time at home to re-read grammar points, rather than letting the lesson turn into a translation exercise. Plus, I can be sure the Italian work-book is correct in its explanations, which would be a pain for me to try and write myself.