Learning grammar is important because it is the foundation of our language. Without it, students wouldn't be able to tackle higher forms of communication. Just as in any subject, more time should be spent at the fundamental level in order to serve as the building blocks for the higher level skills. The building blocks of the English language (e.g. parts of speech, sentence structure, punctuation) should be well understood by any student who endeavors to excel in their communication skills. One must learn how to put words together to build a sentence, one must learn how to put sentences together to form a paragraph, and one must learn to put paragraphs together to form essays, short stories, novels, letters, speeches, etc. Since the foundational level is so important, the most time should be spent there until mastery of grammar rules and conventions is achieved. Only after achieving mastery can one move on and be successful in higher forms of communication.
Learning grammar is a waste of time because we automatically learn grammar as we learn a language. A student doesn't need to know what the parts of speech are in order to form a coherent sentence; they only need to have a solid model to follow. The foundation of English and communication isn't in grammar, it is in the speaking, listening, reading, and writing skills. Students should spend more time practicing these skills, rather than wasting time on confusing and pointless grammar rules which only serve to impede their progress.
[P1] Grammar is the foundation of the English language. [P2] A significant amount of time must be spent at the foundational level if students are to become successful at higher level communication skills.
Rejecting the premises
[Rejecting P1] Grammar is not the foundation of our language; speaking, listening, reading, and writing are. [Rejecting P2] A significant amount of time needs to be spent on practicing the real foundational skills in order for students to advance and master the art of communication.