Suppose one spammer has your email address--that doesn't seem so bad. Well, it can be. For one thing, if you try to "unsubscribe" to a spammer's bogus messages, they know from your reply that there is a real user at that email address that uses that account and actually responds to some messages. At that point, you have been verified, and the spammer may send many more messages to you, ones that appear totally unrelated to the original spam that was sent as a test. Even worse, spammers often trade or even sell databases of "verified" email accounts to other spammers. Your address can be spread around and the number of spam emails you get will increase exponentially.