I know what you're thinking. Every visitor is worthwhile. Every visitor is sacred. The more traffic I can get the better. All these things are true.
Every visitor costs money.
Whether or not these costs are immediately visible to you or are in the form of sweat equity and some low priced service providers, you are spending every minute your site is online. Every e-mail that flows into your inbox uninvited slows down the global network and costs time, money and resources. Multiply that by billions of unwanted e-mail messages and you've got a serious problem. The same goes for every individual web site. Every visitor has the potential become a customer, or contribute to income generation.