Charon121: I don't buy the excuse of "I can spell perfectly, but I just can't bother to make the effort right now" because you either spell correctly or not. Spelling is not something that requires effort or concentration. If you're proficient in the language, it should come to you naturally. In fact, it takes an effort to misspell things if you really know the language.
I think it takes some effort to become pro-efficient enough in the structure and words of a language to write in a legible manner.
Once you have invested the effort, yes, writing properly to be understood comes naturally (and depending on your degree of investment and natural aptitude for linguistics, you may still make some mistakes when you write, but given that the remaining 95%+ of what you write still follows the conventions of the written language, the reader's brain can auto-correct the mistakes).
In the case of my friend, he treated school like a joke during those formative years when you learn proper grammar and write essays to reinforce what was learned.
Afterwards, he never bothered catching up on the lost years. He got a blue collar job so he doesn't need it to earn a living. Overall, it wouldn't be so bad, except that occasionally, he writes and I have to read and understand what he writes.