The recipient's attention and focus are interrupted. The most valuable and precious resource any institution possesses is the consciousness of the humans within it. Successful people and successful institutions value the focus of that consciousness. One reason they have learned to prefer email to, say, a ringing telephone, is that email is less distracting, allowing that focus. Junk mail threatens to destroy this key advantage of email.
Junk mail damages the community.
Since people now know that contributing to a news group, posting to a discussion list, or listing their email address on a web page is opening themselves to junk mail, people are becoming more reluctant to participate in the Internet community. A great commons is being lost to vandals. Like our city spaces, which have become deserted because fear has driven us into secured compounds, the open spaces of the Internet are in danger of becoming off limits to most users.