"Write what you know"

You probably heard that one before: when you want to become a writer, write what you know. I never agreed with it, and I could write a long essay debunking this claim, but I think this line by scifi writer Joe Haldeman (whom I never read), sums up perfectly in a single one example why this is a rubbish advice: "Bad books on writing tell you to "WRITE WHAT YOU KNOW", a solemn and totally false adage that is the reason there exist so many mediocre novels about English professors contemplating adultery." That's spot on.