I would endorse this as well - grammatical texts are useful for refining your understanding of the structure of a language.

Tentatively-- grammatical texts have a complex relationship with language. They can be somewhat useful but still go astray because they're for a different language, with the classic example being grammar based on Latin being used to occasionally force English out of its normal use.

I suspect the same happens when formal grammar is used to claim that casual and/or spoken English is wrong.