[evanpw]

It's possible for a sentence to be understandable, but still be jarring to read. His user page gives another example: "could of". If you see that phrase, you know what the author meant, but it interrupts the flow of reading. There's also the issue that an article (especially on Wikipedia) seems less credible when it contains spelling and/or grammatical errors. It gives the impression that the author wasn't careful when they wrote it, or that the article hasn't been widely reviewed by others.