I'm guessing you meant this rhetorically, but I'm going to answer it as a serious question , since it's so central to at least one area of linguistics (syntax). Not all 'bad grammar and spelling' is created equal. I'd break 'bad spelling and grammar' down roughly into these categories:

(1) Non-standard orthography: In other words, "spelling and punctuating things wrong." Any orthographical system is going to be arbitrary to some degree, and if you don't follow the standard one, it implies that you're not very well educated. I don't have a problem with this. (But, as a linguist, I have to point out that a lot of interesting phonological information is lost when writing systems don't change to match the spoken language, and then often the only information you get is through misspellings. For instance, one of the best sources on how late Latin was pronounced is a handbook correcting common spelling mistakes.)

(2) Un-English grammar: In other words, sentences no native English speaker would ever produce (like "I pick up coffe mug"-- you simply have to have an article before "coffee mug." Again, it implies that your knowledge of the language is bad, and I don't have a problem with this, either.

(3) Failure to follow rules of English prescriptive grammar: A lot of these rules are completely idiotic and have nothing to do with the way English actually functions. Examples include banning singular "they" in favor of gender-neutral "he," and dangling prepositions, splitting infinitives, banning "which" at the beginning of a restrictive relative clause, the rules governing when "whom" should be used. In fact, there are some pretty easy linguistic tests which demonstrate why the first two are simply not wrong in English. Um, as you can tell, I'm very passionate about this one...

(4) Real English sentences of questionable grammaticality: An example would be "Is this the boy who she asked you whether he should come?" If you survey a bunch of native English speakers, some will say it's fine, others will say it's not, and still more will be very confused. It's not clear what's going on, but we would probably have a much clearer idea of how the grammar of English works if we actually had an answer to that. I would probably call the sentence stylistically bad and move on with my day.

Anyway, that's probably more than you ever bargained for

Also, isn't the subordinate clause here missing a subject?

There's draft writing and there's editing. A few people are skilled enough to say precisely what they mean and get everything spelled right and grammatically correct the first time. The rest of us rewrite and edit -- or not.

Failure to edit could be a result of mental or physical laziness, lack of manners (it's polite as well as prudent to make something other people are going to read -- readable), or being in a too much of a hurry. It's sometimes hard to tell.

In schools grammar and spelling are often taught as a separate subjects rather than as tools one uses to improve ones writing in the editing process. Probably many people haven't learned to edit their own work (beyond perhaps running a spell-checker).

Whether mistakes affect my opinion of what a person says depends on a lot of things. I always appreciate clarity and a level of formality that's appropriate to the subject matter (and medium). If the topic is serious and the writing looks like a dashed off first draft -- I supppose I regard it as such. Doesn't mean there's not a kernel of a good idea there but I may not take the time to sit and think about it.

These days a lot of mistakes in a blog could just mean the author lives in a drafty house back East and can't afford to turn the heat up.

Will most people judge you differently if they know you are not a native speaker? I feel I am making very little spelling errors but I get the grammar wrongly frequently. I mean like the example with the article. It is not so clear when to use it and when to omit it.

Note that there are a few Facebook groups like that: I judge you when you use incorrect grammar, spelling, and pronunciation.

To the previous poster:

I can usually tell if a person's grammatical mistakes are the result of their not being a native speaker. I spent two years teaching English to college students in Taiwan while beginning to learn Chinese, and I generally could tell why my students said things the way they did. (I recognized what Chinese pattern they were translating word for word to English, why the distinctions among "he," "she," and "it" didn't come naturally, why although I'm female some students called me "sir" and the rest called me "teacher").

I'd think anyone who has made a serious attempt to communicate in a language that they did not grow up speaking would understand what an accomplishment it is to make sense to a native speaker, much less to get all the grammar right. I can't tell you how a random person on Facebook would judge you, but I really wouldn't worry about a stranger who held a misplaced article or two against you. On the other hand, if you're writing for publication in English, it would probably be wise to have someone more fluent than you read a draft to catch just the kind of mistakes you have trouble recognizing yourself.

(An aside: I tried to read a draft of a paper for a friend from Hong Kong, but it was almost impossible to correct. My friend was a philosophy professor writing about Martin Heidgger, whose work he'd studied in Germany, in the original German. Heidegger was known for stretching the rules of his own language, and it was almost impossible for me to tell what was bad English vs. well-translated Heidegger. I mostly held this against Heidegger and not my friend.)

Blue Morpho