I can understand both points of view in this thread honestly. As an English language and literature student (previously), I have to admit I find things like not bothering to differentiate between 'too' and 'to' annoying. I don't object at all if it's someone with dyslexia, or speaking English as a foreign language, because they're completely valid reasons to not be able to differentiate, and I'm sure there are plenty of other valid reasons such as perhaps having a mental disability. I only object when it's people who have had the opportunity to learn, and then continued to make the same errors.
However, I have a condition which causes cognitive dysfunction, so I will occasionally make the very same mistakes that annoy me when others do it. It's a progressive condition, so I'm only going to get worse and start doing it more. It's why I'm trying to make less snap judgements about people (and their spelling/grammar!)
To speak in chat/text speak on a forum that specifically requests you don't, and to have truly poor posting etiquette (no breaks, full stops or commas etc) when you know there are people who may have dyslexia, or not have English as their first language and would have trouble reading your posts, is just inconsiderate in my opinion. But only if they're capable of typing coherently and then choose not to.