In today's world, it is almost impossible to find absolute beginners in most regions of the world. Even young learners come to classes with some English! We're surrounded so much with English and it's nearly impossible to find beginners who know no English at all. They know English before your input!
In the world of ELT, prescriptive grammar has a special place among teachers and learners. On the other hand, descriptive grammar is perceived as an approach which only linguists are interested in. Although descriptive and prescriptive grammar approaches have clear definitions and leave no doubt on their distinctive uses in the ELT world, their reflections in ELT classes have debatable consequences.