>that is not true at all - not knowing grammar well doesn't mean the writer has nothing good or creative, or worthwhile to say (or write about).
Having something to say and saying it effectively are not synonymous. There are many people with worthy ideas who fail to express themselves competently.
I have yet to encounter a good writer who has poor grammar. Good writers care about grammar, for the same reason good programmers care about style: it's a mark of careful and deliberate artifice.
A good programmer doesn't simply want their programs to compile and run without error; they want it to be readable, maintainable, and elegant. They want the quality of their code to speak for itself. Similarly, a good writer seeks not only to clearly express their ideas, but to do so in a way that evokes thought and emotion on the part of the reader. To do so it is necessary, but not sufficient, to achieve technical mastery of the language in which you write.
Good carpenters don't make sloppy cuts, good chefs don't burn the rice, and good painters don't make stray strokes. These are the easy things to get right: the bare essentials for being competent at your task. A person cooking a meal for their family may use too much salt, a person playing guitar around a campfire may play the wrong chord, and someone responding to a comment on the Internet may talk lik dis,, or use unnessary punctuaton!
The difference between the craftsman and the lay person is that the craftsman takes as writ the technical mastery of their skills, and focuses on composition. The lay person is concerned only with the gist.