I don't know if it's so bad. In textual communication, we don't have inflection, body language, or many of the other elements that can imbue a face-to-face conversation with certain meanings or tones. One way to convey a friendly tone can be the employment of loose-yet-intelligible grammar, spelling, and punctuation in your messages; it assumes a certain degree of familiarity and can dissuade notions of stiffness and reinforce notions of laxity.
In some situations, the formality of exact punctuation and grammar can be unsettling. Perhaps discussion of something as business-like as stock options is not the time to drop the formality, but it isn't necessarily a sign of disrespect; in fact, it could be the opposite.
"Loose-yet-intelligble" is exactly the kind of grammar you have used in this comment, though.
You should have put a comma after "this" in your first sentence, and you've got a comma splice in the second sentence.
The point they were making is that violations of formal grammar can be useful in some contexts and for some purposes.
What I just wrote is an apt example, actually. I used "they" as a gender-neutral third-person pronoun because, though it's not standard grammar, it suits the tone I want to convey here.
This is a silly game, you mispelt "intelligible". The missing comma is well spotted and down to my lack of proofreading, the comma splice is more stylistic than anything in my view.
However, by concentrating on how I wrote, rather than what I wrote, you are reinforcing my view that "loose-yet-intelligible" is not OK. You provided further reinforcement by writing a clumsy sentence which you then felt the need to justify.
I think it depends on the context, as above. If your friend that you know is well-educated sends you a message with loose grammar, are you going to go on thinking he's retarded now?
I don't do this personally, just taking a potential angle on the letter. It is interesting that some people are so judgmental and stringent about this, though.
It doesn't matter how well-educated you are - if you can't be bothered to write a coherent sentence and take a couple of seconds to make sure you're using capitalization and punctuation correctly, why should we believe you're going to be careful in other areas of your work? You're allowed to make mistakes, but not caring about how you write would make me worried that you're more likely to be lax in other areas that are even more important.
In some situations, the formality of exact punctuation and grammar can be unsettling. Perhaps discussion of something as business-like as stock options is not the time to drop the formality, but it isn't necessarily a sign of disrespect; in fact, it could be the opposite.