> none of these things have any impact on the ability of a reader to understand the writer's intended message

I disagree, I've encountered several instances in corporate communications where the use of then instead of than has completely changed the meaning of a message.

Imagine writing a compiler that spent much of its parsing time trying to infer what the programmer meant, instead of enforcing a syntax and getting-on with the task of translating to machine code. It would be an enormous behemoth that would take much longer to do its job than necessary.

Yet, strangely, those programmers who don't care much about grammar seem quite happy to adhere to arbitrary programming language syntax. Why is that? Why don't they force compilers to just accept their possibly-incorrect inputs with the excuse 'you knew what I meant'?

I agree with you. I am seriously annoyed by the kind of mistakes you've listed, and it has nothing to do with the message being understandable or not. Instead, it means to me that the writer (often a native English speaker) has never bothered to master even the most basic rules of his own language, which in turn suggests a pretty poor education, at least as far as reading and writing skills go. Also, since he has never noticed his mistakes, he's either poorly literate, or displays a remarkable lack of curiosity and observation abilities.

On the other hand, I'm Italian. I came to understand, after a few years living in an English speaking country, that the emphasis Italians put on writing correctly (as well as on many other things - e.g. eating well, dressing decently, avoiding public drunkenness) has no correspondence in the English speaking world.

Did I annoy you enough? :)

English-speaking countries are not all alike. The level of non-English speaking immigration to a country (which happens in all English-speaking countries, but to a much greater degree in some than others) has a large impact on the ability to maintain high standards for the language.

As I'm sure you know, English spelling is also much less phonetically obvious than Italian (and I would not be surprised to learn that English is slightly grammatically more complicated than Italian, although frankly, as far as I'm concerned, any language with grammatical gender wins for difficulty), so emphasizing correct writing might be slightly harder.

I don't think it's a matter of immigration, I am living in a country with a relatively recent immigration and a lot of natives are making horrendous mistakes (usual suspects: "there" for "their", "who's" for "whose", "definatly" for "definitely", etc.).

You second point is absolutely correct: English spelling is a nightmare especially if you don't have any alternative phonetic system in which to represent it. My Italian wiring makes "there" and "their" two completely different words (they would be pronounced differently if read phonetically in Italian: "ther-a" and "the-eer") - English speakers don't have the luxury of a discriminating phonetical representation. By the way, this whole thread is about "grammar pedants" - but in fact we're just talking about spelling. The construction of the sentences is very rarely wrong.

I tend to find it easier to hold a discussion with those that say what they mean. I personally have found the type of person that says "You know what I mean." in response to corrections of the "they're/their/there" sort to be intellectually lazy in many of their endeavors (and in support of the one of the article's premises, typically "outgoing" as well).

The type of person that sprinkles their text with whatever word seems appropriate enough and leaves you to figure out what they truly mean is offloading their cognitive work onto you, to the detriment of communication and understanding.

They don't impact the ability of the reader to eventually understand the message, but they do decrease the speed at which you can do so if you read any faster than purely phonetically. When your reading begins to involve visual pattern recognition at the word level as well as some degree of pattern recognition of higher-level structures, an error like that is an annoying distraction.

For writing where more time is spent reading it than typing it, it indicates inconsiderateness not to apply the necessary amount of spelling and grammar checking if the writer is unable to produce well-formed writing without specifically attending to its well-formedness. (Although it could also be a status signal to be able to make errors.)

(It's the same reason for code formatting and style rules - they standardize the low level details so readers can focus on the higher-level meaning.)

Any native English speaker who reads much at all (not to mention writes!) and has above a certain level of innate linguistic intelligence will also automatically develop perfect grammar and perfect or near-perfect spelling, so making frequent errors also indicates a certain semi-literacy.