Just like a carpet cleaner shouldn't leave behind too many spots, a good writer should have clean copy as well. Think of typos, grammar mistakes, and spelling errors as the spots on your virtual carpet. No one is perfect. But if you are going to write for a living, you need to make sure that your copy is as clean as possible.
Do you think someone is going to hire a carpet cleaner that leaves behind some spots or one that's known for getting out most or all stains? Web writing is the same. You are the expert and your finished product should demonstrate that.