The main problem with most EFL textbooks is that they teach standard English when most English speakers us non-standard lexico-grammatical forms. All of my advanced students ask me why I'm teaching them an abstract form of English which bears little or no relation to the form of English they see and hear on the street. Has anyone else experienced this?
This is a dilemma that anyone learning a foreign language will face. I think there is a problem with EFL books that solely teach an "abstract" form of English that will not help these students when using the language in their daily lives. However, it's difficult to figure out what form of English to teach in a book. For example, going back to the discussion on double negatives....
Sure they're used by many English speakers, but like it or not, in many cases there is a stigma that goes along with them. I don't think the teacher would be doing the students any favors to teach them that "I ain't got no money" is just as socially acceptable as "I don't have any money" or "I haven't got any money." The meaning is the same, but there is a social reality here. Students need to know when certain types of phrases are appropriate and when they're not. It's best to teach students what is considered "erudite" modern English. Slang, idiomatic expressions, dialectal differences etc. are all necessary to know and should be included in any good textbook, but there needs to be some sort of standard to refer to or else it'd be too much to teach.
Then you wander way into that territory of adapting your style of English for a particular context, right?
If I were learning English from a basic level, taking into consideration things like who I'm with/where I'm at/what point I'm trying to make would probably completely overwhelm me. Since you've got to start somewhere, a standard form is probably as good a grounding as any, and then you go on from there. It's a little bit like that spelling reform from an earlier thread; it would be nice to change it to something better, but what is an ideal model?