Descriptivism is a philosophy in linguistics that says that linguists should describe how people use language, and not attempt to prescribe or proscribe how they should use it. As such, it rejects prescriptivism
Nearly all modern linguists are descriptivists, not prescriptivists.
While useful for the academic study of the language, and for EFL teachers in particular, such grammars are perhaps of little direct use for non-native language learners who may not really interested in a statistical analysis of what native speakers say, but want a prescriptive grammar which gives clear rules about what is acceptable and what is not.
Nevertheless, teachers should be wary of personally accepting as gospel all the simplifications included in coursebooks and would do well to remember that they are exactly that - simplifications to help non-native speakers. (see pedagogic grammar v. scientific grammar).
There is also not quite a dichotomy between descriptivism and prescriptivism. Most descriptivists find it acceptable to note traditional prescriptive rules, and warn others that they may be judged for not following those rules, while demonstrating that speakers routinely break those rules.