I was talking about this once with a programmer friend of mine (I'm a programmer too btw). We used to get letters to set up in the automatic system and they often had quite appalling spelling or grammar in them, even though they had been supposedly 'proof-read' by the departments producing them. We realised that (good!) programmers have a very good attention to detail, because the slightest typo or semi-colon out of place can prevent a program from working properly. I also studied languages before becoming a programmer, which means again that I am used to paying close attention to grammar, spelling and punctuation.
It's just the kind of guy I am! All these things improve communication of the written word. I'm not that bothered about things that are wrong in informal things like posts and emails, but when official documentation is produced I think it looks very bad when mistakes are made.