The Argument
Grammar is learned intuitively as one learns language. It is a waste of time to study grammar when that time could be better spent on developing better communication skills in reading, writing, speaking, and giving/receiving feedback. As the student learns through practice, they will automatically pick up on any grammar nuances inherent in the language that they are learning. One does not have to know what a grammar rule is to be able to use it.
Instead of spending time on grammar, students could spend that time learning how to read and studying literature. Literature serves as a model for how to write clearly and effectively. The more students are exposed to examples of good writing, the better at writing they will intuitively become.
The same goes for speaking: if teachers were to spend less time on drilling grammar rules and more time on allowing students the chance to practice speaking, then students would develop better communication skills. This does not have to be limited to presenting in front of the class; it could include group work or pairing students off in order to practice discussing literature and writing with one another.
Another skill which is severely lacking in today's students is the ability to give and receive feedback. Feedback is an essential communication skill that is highly important in the workplace where one needs to know what they are getting right and what they need improvement on. Many students lack the skill to gracefully receive feedback, and they also lack the skill to respectfully give it. Instead of being quizzed on grammar, students could participate in workshop groups where they practice giving and receiving feedback with one another on written or oral classwork.
The number of essential communication skills which are being passed over in favor of studying grammar is astounding. Teachers should focus on teaching these skills instead of drilling students on a facet of language which is already learned intuitively through practicing these skills in the first place.
Counter arguments
Grammar can be taught in conjunction with these essential skills; it does not have to be one or the other. It is important that students have the vocabulary to talk about language, and grammar is the language they need in order to do so.
In reading, students can talk about why a particular author's work is effective if they know what grammar conventions the author followed. In writing and giving/receiving feedback, students will have a better understanding of what to look for to improve in their writing if they have the vocabulary to talk about the grammar rules they need to be following. In speech students can grade one another on how well they understood their classmates' speeches based on whether the speaker used proper grammar or not.
Grammar is an integral part of language learning and should be interwoven with other essential communication skills, not segregated from it. If incorporated in the lesson properly, it would take no extra time to learn the grammar rules alongside the essential skills.
Premises
[P1] Learning grammar is a waste of time.
[P2] The time spent on learning grammar could be better spent on more essential communication skills.
Rejecting the premises
[Rejecting P1] Grammar study is an important part of learning language.
[Rejecting P2] If grammar study is incorporated into the lesson plan properly, it would take virtually no extra time away from learning the essential communication skills.