Big side discussion solely for ganjavih here...
When Linguists refer to "language", they are referring exclusively to spoken language. It is speech which is natural and unavoidable (i.e., you cannot stop a child from acquiring language; the human brain is hard-wired to produce it) - and spontaneous. Thus, all of the principles I referred to are relevant to speech.
Writing systems are another matter entirely. They are the opposite: unnatural and invented phenomena. If your language does not have a writing system, you need to borrow, steal, or invent one. This is why, while all spoken language is equal in complexity, all writing systems are far from equal. They range in complexity from Spanish using the roman alphabet, in which there is almost a 1:1 correspondence between letter and sound, to English using the same alphabet but with far from a 1:1 relationship (26 letters and more than 40 sounds) to the Chinese system of symbols in which a person is not considered literate until perhaps 20 years of age.
Once Spanish children know the alphabet and the sounds associated with the letters, they can read anything, even if they can't understand it; it will just take time to increase vocabulary and learn what the bigger words mean. But children learning to read/write English have to keep learning exceptions to rules and the different spellings that come with the history of the language and writing system, which reflect that English has embraced borrowings from every language it has come into contact with.
So writing systems are completely rule-governed. We need to follow those rules for the most part, to maintain a semblance of order in a writing system such as English. If you've seen that joke that gets passed around through email every so often, proposing that the English spelling system be simplified, it's amusing but in all seriousness a bad suggestion. It would lead to all kinds of confusion resulting from the many homonyms English has that are disambiguated by spelling.
Taking an example from my talk: the use of "ain't" in speech is in no way inferior to the use of "isn't". This is a dialect difference. But as an analogy: the misspelling of "its" in print is not an indication of dialect, community, or any other social factors. It is either an indication of sloppiness or a lack of knowledge of the rules of one's own writing system.
These days, there are many areas in which you can avoid writing, and many people do avoid it. Unfortunately, unlike speaking, which we all do from an early age, writing is something some have a special talent for (some people really are naturally good spellers). We do acknowledge that in some situations (e.g., resumés and job applications), a single mistake can be disastrous. In other places, such as on this forum, a more casual level of writing is accepted, but owing to the higher-than-average level of discourse, you won't get the warmest reception if u rite like a 15 yr uld talkin to frenz on a cell phone.
This whole thread (and these regular discussions) are not chastising those who really have genuine trouble communicating. They are good-natured, but bring up spelling/grammatical points which the vast majority here seem to be capable of actually learning - if they don't know - and incorporating into their writing.
Addendum:
In speech, nonverbal cues (tone, intonation, facial expression, body language, to name a few) help to clarify meaning. Writing, without the aid of any of those, is very easily misunderstood. That is why spelling and grammar are critical in helping to disambiguate.