Obviously, I'm biased here. It isn't known to the first, second, and third -ime reader that I'm not a native-speaker and a lot of my points also suffer from grammar not too brilliant. Additionally, I fail at making use of Grammarly (and so do the text entry fields on both Busy and Steemit btw).
Thus it won't be a surprise that I disagree. The grammatical credibility argument is a form of elitism which can not be beaten quick enough, especially not on a democratised platform like the Internet. Not because we shouldn't strive to craft better, more enjoyable to read prose (there's a reason why I read the Guardian and enjoy especially the sports pages) but because the way the grammatical excellence and [sic] stick are used.
The trained, analytical reader will long have observed that both are only used when people disagree with the writer's opinions, never when they share the same wave length. Yet, in a democratic society we shouldn't let the literary skills of one define whether their opinion is valid, let alone should be given credibility. Each opinion matters, just like every adult is allowed to vote (and we should most definitely also tend ear to adolescents as their future may depend on our actions and decisions).
To argue that one's education, ability to dedicate sufficient time or capability to proofread and correct themselves may dilute their thoughts and opinions is shallow.
Yet, I wouldn't be myself if I weren't to admit that as self-confessed closet nerd professional reader the Internet was a better place when everyone still needed to know FTP, HTML and CSS to spew their stupidities all over the world wide web but it would be hypocritical to stick to such position and dismiss the democratization of publishing. Together with which comes the validity of everyone's thoughts, no matter whether we agree with them or not.
No matter whether they are literary as skilled as we love reading.
Yet, poor grammar can easily be beaten with [cmd]/[windows] + [w] and often is on my keyboard. And probably is almost as often by first-time readers of my crafted content. Even though I know the difference between you're and your as well as they're, their, and there. :D