Some of us love language for language' sake, and for those of us who suffer from that love, bad or incorrect use of language hurts. It's not just being picky or stuffy or annoying - it genuinely does hurt.
Gentle reader, try to be tolerant. If the author is here, surely it is because they wish to learn, and how can they learn without guidance?
Writers: think about it. At the simplest level, if you mean 'red' you don't say 'blue'. So: they're, their, there; I, me; your, you're: they mean different things! How can you say what you mean if you don't know that?
The written language is the writer's tool, and I believe we should all learn to use it as part of learning our writing skill. What use is imagination if you can't describe what you mean and build it in the reader's mind? How much more powerful and evocative can you be, when you have control of language as a weapon?
You have no idea how wonderful being able to communicate is until you can't do it! (I have lived in a foreign country and worked very hard to speak/write the language; I love it and miss using it. Sadly I now misuse it).
Of course, we all make mistakes, proof-reading your own work is really hard and yes, this is for free! But there is no excuse for pure laziness (I do forgive over-enthusiasm - I have to, it is one of my major sins! That and being verbose).
At the same time as supporting good and loving use of language, though, I believe we should all also fight against unreasonable shackles of correctness: why aren't I 'allowed' to start a sentance with 'however', and various other words, when that is exactly what I mean? (No, please don't tell me, it is rhetorical!)