As someone who has studied Spanish since the age of thirteen and has lived in a predominantly Spanish-speaking city for many years now, I have quite a bit to stay about language textbooks, Spanish language textbooks in particular. Most of the Spanish language textbooks I have encountered (and I have run into a lot of them, believe you me) teach an artificial type of Spanish that really only exists in Spanish textbooks themselves. When I moved to Mexico City at the age of sixteen and spoke what I would characterize as a fluent but "synthetic" Spanish, my speech was greeted by a fair share of giggles, although they were probably caused by surprise more than anything else. When I returned to my small-town Virginia high school a year later, my Spanish teacher (a lovely lady and a truly caring teacher from Illinois who was drafted into the job because she minored in Spanish in college) could barely understand me at times.
Although commercial textbooks do have their benefits (in most of my junior high and high school Spanish classes, the textbook was the course syllabus and some of the teachers would have been completely lost without them), they pale in comparison to authentic materials because only authentic materials can teach students the real language, with its unanticipated twists and turns, its less-than-literary but extremely useful constructions, and the type of language that would come in handy should learners ever have to use the language to communicate with native speakers. I remember once in eighth grade Spanish (I had the great fortune of having a passionate and phenomenal teacher that year, as I was living in the western suburbs of Pittsburgh which have some very high quality public schools) that we spent several classes watching commercial advertisements from Latin American television networks that the teacher had videotaped during her visits to South America, and I was amazed at the difference between the language I heard in those commercials and what we had been studying from the textbook. It was almost as if the two were entirely separate languages.
I also remember some of the utterly useless language that was presented in our Spanish textbooks. One of the first Spanish words I learned was pupitre (those individual desks about two feet from the floor that are used in grade schools), which unless you happen to work in an elementary school or a furniture factory is a word that you might use five times in your adult life, if that. Then there was the whole lesson on cuyo (a rather formal way of saying "whose"), which I have only seen in legal documents and is rarely used in spoken Spanish. Conversely, in all my years of formally studying Spanish, I amazingly never saw a single lesson in any textbook on the use of the reflexive pronoun se as used to form medio-passives or the so-called middle voice (unlike English, which just has two voices--the active and passive--Spanish has three voices: active, middle and passive), as in Aquí se habla español ("Spanish is spoken here") and is extremely common in both spoken and written Spanish, so common in fact that one cannot really speak Spanish fluently without being able to use the construction effectively. So, to sum things up, I would say that language textbooks are useful and even necessary in certain circumstances, especially in instructional contexts with weak or marginally-competent instructors, but that they are absolutely no replacement for authentic materials, which are crucial for learners to acquire real (or as I like to put it, "non-synthetic") language.