In arguing for the use of formal grammar, some proponents say that formal grammar improves students' writing skills. Formal grammar gives students a better understanding of the language and of the corrections that others may make to their writing. However, others argue that formal grammar does not improve writing. Some studies have to come to the latter conclusion, but some people have questioned the methods of those studies.
One major strength of formal grammar is its universal communicability. Formal grammar will not be misunderstood by anyone who speaks the language. It can also be used and understood over a long period of time, because formal grammar does not change over time like slang usages do. For these reasons, it is easier to learn subjects using formal grammar. Additionally, a strength of formal grammar is its testability. Since formal grammar is based on an objective set of rules, it can be tested to show whether students have learned language.
One problem with formal grammar is that, while formal grammar itself is objective, language is subjective. Formal grammar needs no context, but human conversations are context-dependent. Different people use varying degrees of formality in different situations or for different reasons. If you remove the context from a situation, the meaning of the conversation is lost. Sometimes informal grammar use is necessary to retain the context of a situation.
Some people argue that knowing formal grammar is not as important as knowing how to write well. These people would shift the focus of learning from formality to communication. Another weakness of formal grammar is that students' understanding of it will lessen over time without continued reinforcement. Similarly, it is more difficult to form associations with subjects taught using formal grammar. Students understand and recall subjects better when they have been taught to them in their own vernacular.