Disclaimer 1 : I make no claims to being an expert in the English language, nor do I claim that my writing is perfect and free of errors. There are many rules of grammar and punctuation that I am not aware of. I am still learning.
Disclaimer 2 : Genuine typos and deliberately used acronyms (E.g.: LOL for 'laugh out loud') as well as short forms (E.g.: sms lingo) are out of scope for everything that is written below.
I suspect that people have always had difficulties with written English, with its non-phonetic spellings and probably the most complex punctuation rules among all languages. This difficulty has never been as obvious as it is today, in the era of Internet fora.
Online fora such as message boards and blogs enable anybody to type and publish stuff, which are almost instantly read by more eyes than was ever possible in human history. These writings need not go through any form of quality control such as editing and proof-reading. The result is that a person visiting a typical online forum can be exposed to more atrocious - qualitatively and quantitatively - spelling and grammar in a single day, than s/he might have come across in a whole month or even a year, in the pre-Internet days. Assuming, of course, that the person is an average reader, and not a magazine editor or an English teacher.
Most online fora have at least one spelling/grammar-nazi - you know, the person (mostly men, rarely women) who points out and corrects spelling mistakes and grammatical errors. Reactions to such corrections range from mild irritation to outright hostility. The reactions are also normally accompanied by excuses.
Two of the most popular excuses are:
(i) "I was quite aware of my error when I was making it. I only made the error because this is an online forum. I would never make such errors in an official report or formal paper."
(ii) "Nobody cares about such things these days. It is not really important to be grammatically accurate and use correct spelling, especially not in my profession/office." This particular excuse, unfortunately, is most popular among techies. Most techies who give this excuse are at the non-management level.
These excuses are not just unacceptable, they are invalid.
Before I go further, it is worth repeating my earlier disclaimer: Genuine typos and deliberately used acronyms (E.g.: LOL for laugh out loud) as well as short forms (E.g.: sms lingo) are out of scope for everything that is written here.
"I was aware of the mistake. I did it only because this an online forum."
Sometimes this is an outright lie, and it is very obvious that the person who made the mistake was not aware of it at all, or else s/he would not have let it pass. Confusions in the usage of "there", "their" and "they're" as well as misuse of apostrophes come to mind. Other times, the person making this excuse is simply deceiving herself/himself. Think of formal writing as attending a formal event and informal writing as attending an informal event. For formal events, you may be formally dressed and for informal events you are informally dressed. However, if you are classy and decent, to neither event would you wear soiled or smelly clothes, nor clothes which are torn and expose your private body parts. In the same way, classy and educated people will not, all of a sudden, lose their sense of spelling and grammar just because they are writing in an informal forum. They would certainly not allow their own mistakes to pass uncorrected, if they were aware of them. Of course, there are always exceptions - one might deliberately use a certain style of writing, replete with errors, for a specific purpose. This is no different from someone who might dress outrageously for an event, in order to make a specific statement or create a certain impression. But those are exceptions.
"Nobody cares. Correct spelling/grammar is not important for my job."
This is a dangerous attitude/belief to have. It is akin to believing that how you dress to your workplace is not really important. Your office may not have a strict dress code, you may be allowed to dress casually daily, and nobody might question you if you were to dress badly. However, if you believe that people are not getting certain impressions about you and not forming opinions about you based on how you dress, you are absolutely wrong. It may or may not be fair to draw conclusions about people based on their dress, but it happens. Consciously or subconsciously, it happens. A similar thing happens with how we speak and write as well - it leaves impressions, causes people to judge us. In fact, how we write might leave a more lasting and widespread impression, because there is a paper or electronic trail. Also because emails might be forwarded far and wide, and online postings are seen and read by many people. If the analogy of dress does not convince you, think of speech. Imagine a person who speaks any language badly, with incorrect grammar, may be in a country bumpkin fashion. And then, imagine a person who speaks the same language cleanly and correctly. All other things being equal, who will leave a more favorable impression? Whom would you consider to be more capable, reliable? With whom would you prefer to trust your business and your money? The one with a better manner of speech, right? Believe me, the same thing happens when you write badly. Bad writing gives the impression of you being less educated and incapable.
There is a subtle trap here for people like Indians, for whom English is not the first language and is most often learnt only in schools and colleges. In most places in India, the very fact that a person speaks and writes English, might make him appear educated, a cut above an uneducated person. This would happen regardless of whether the person's English is grammatically correct or not. This might lull a person into a false sense of confidence that simply being able to write in English, however badly, is sufficient. The person might not realize that in a country like America and in online fora, his/her bad grammar and spelling might make him appear to be uneducated. Non-native English speakers must be careful about this.
Let's face it: the only person who can get away with bad dress, bad speech or bad writing is someone who is already extremely successful, powerful and/or beautiful. And even such people become the subject of ridicule and jokes, if their language skills are bad (Shrubbish.com). Unfortunately, the majority of us are not in that state of extreme exaltation. For us, it is important to dress, speak and write cleanly. We don't need to go to extremes or be experts. We don't need to dress like movie stars or write like professional writers. Just being able to write in simple sentences, free of spelling and grammatical errors should suffice. It is not easy to do that in a confusing language like English. But it is not too difficult either. Most often, it is merely a matter of reading more, learning better and being more aware while writing.
The most important reason for writing right is not just to create a favorable impression of ourselves to the reader, although that is important. The most important reason is simply to communicate right. Bad language and errors dilute and distract from the message which is sought to be communicated. Moreover, anything worth doing is worth doing well. An apt word used right in a well-constructed and error-free sentence is a thing of beauty worth striving for.
Related: Punctuation is Dying