Bad spelling, punctuation, and grammar are like hammer marks: you can take the time to clean them up or you can decide that a few aren't a big deal, but if there are too many, it just looks careless and sloppy.
Furthermore, as Michael Cochran suggests, it's a courtesy to your reader. A pretty fair number of my posts and comments are entered on my smartphone, and I consider that if I want someone to invest time in reading what I have to say, I'd better take the time to make it readable. The Edit button is a wonderful thing.
(That said, I am willing to give some leeway to the many IFI members whose native language isn't English, like the Australians.)