Good question. Some ENTJ's too.
...could be a 'J' thing..?
Well J's in general tend to have preference for order and structure; grammar provides a specific structure to build language around so that it makes sense.
I don't care much for spelling, but grammar I do find has rather important value; it coordinates things in a way that makes sense. While it doesn't matter if yeu say cat or kat, people know whot yeu meant. If yeu say "The cat chased the ball of yarn." then yeu know it's a full sentence, that it has a main object, and a secondary object, and that they interacted somehow, and there is purpose behind it. "ball chased yarncat" doesn't make nearly as much sense, especially with lack of punctuation, even moreso if it's dumped into the middle of several run-on sentences which lack punctuation to distinguish the end and starting points. Even in and of itself, it doesn't really make alot of sense, and would normally be interpreted to have reverse meaning of the original context.
That being said, as long as the message is gotten across clearly, grammar can once again be sacrificed, same as spelling. Grammar, however, tends to be far more important towards creating a sense of context from which one can determine whot is being said. Yeu can use made up words, or total nonsense in a sentence, and it can still make total sense to those listening, simply by means of the context its' used within. Remove the grammar and such context is lost, leaving one without a basic structure to attempt to make sense from. Yet, even so, grammar itself isn't always required, either, and sometimes can even be detrimental.
Yoda, and lolcats, may irritate some people to the core, yet at the same time, they can also provide some rather interesting insights at times, despite the awkward sentence structure.
I think the bigger issue, as a whole, comes down to the fear of looking unprofessional, though. That a lack of order in speech, or text, somehow automatically means an inherent lack of order, and therefore logic, in the reasoning itself.
Such is not an accurate comparison, and really needs to be discouraged, as it just isn't true in the vast majority of cases.
In the end, language is a means to an ends; the means to describe clearly one's thoughts and ideas, and present them to another in a method with which the listener can understand.
One can have absolutely perfect, flawless spelling, with precision grammatical skills... and yet fail to describe even simple concepts. In contrast, one who has no capacity for speech at all, can often be quite clear as to the message they wish to convey. Be it a cat, a baby, or a deaf person, they can still get their message across quite clearly most of the time; language just allows for more intricate details and specifics is all.
Anyways, grammar has its' uses, far moreso than spelling, but still, it does not trump the message; the importance is the communication of the idea, and specifically, that the idea itself is conveyed. So long as this occurs with relative ease, the methodology by which such is transmitted, truly doesn't matter.