Geoff Jordan: "[In] most English language teaching […] students are led through units of a coursebook, spending much of the time working on isolated linguistic structures and carefully-controlled vocabulary in a sequence which is externally predetermined and imposed on them by the textbook writer.
Research suggests that interlanguage development progresses in stages and that it’s impossible to alter stage order or to make learners skip stages. Thus, teachability is constrained by learnability and any coursebook-driven syllabus which attempts to impose an external linguistic syllabus on learners is futile: learning happens in spite of and not because of the course design.
[According to Mike Long] : "Controlling grammar, vocabulary and sentence length results in a limited source of target-language use upon which learners must rely in order to learn the code. The often tiny samples are worked and reworked in class, whether practiced until rote-memorized, milked meta-linguistically, or both, and learners are expected to learn the full language on the basis of access to such limited data”.