Remember that particularly nasty college professor who threatened to dock a letter grade from your papers for every misspelling he found? What was his problem, anyway? Shouldn’t the content of your writing matter more than a few spelling mistakes here and there?
Like it or not, your grouchy prof was trying to teach you a valuable lesson. In writing, image matters. Sloppiness reflects poorly on a writer and, by extension, whatever argument the writer is trying to make.