[The use of] grammar is not an indication of intelligence, and I don't think it was intimated as such in the previous posts. Good grammar is applied to enable
understanding of the written word.
If someone is unable to write properly, then it follows that they will not be understood correctly by the reader, which then leads to misunderstandings, often through ambiguity. e.g.:
a) Gladly, the cross-eyed bear.
b) Gladly the cross I'd bear.
Where a) refers to a bear named Gladly who needs spectacles and b) refers to someone who'd happily carry a heavy load. Both sentences sound the same, but mean completely different things and are written to illustrate their meaning!
Human beings alone are born with the natural ability to read, write and articulate language. Being educated is not necessarily the same as being intelligent. Being educated to an acceptable level in order to communicate effectively should be the goal of educational establishments, like schools.
Children will learn what they are taught, good teaching or bad.

CC x