To the remarks by Jutta, I'd like to add just my perspective as a system administrator. Because in addition to the problem of the data volume that accumulates (and which many still pay for based on data volume), there's another problem: bounces on bounces. The result of "contaminated" address lists is that many emails bounce. Those that bounce to forged but real user addresses land in that user's mailbox and annoy them. The others that cannot be delivered usually end up as corresponding system bounces in the mailbox of the system administrator of one of the involved mail servers.
I'm quite directly affected on several systems where I'm admin - the result is often unreadable bounce mail folders because so much flows in that you don't really want to wade through it to search for real problems. As a result, system problems that lead to mail delivery issues are often only noticed when users complain - nobody looks at the mess of bounce mail before that, just as little as the error reports of the mail server.
So if you like your system administrators (or simply want your mail to be handled properly), then think about them too when you carry out such anti-spam actions.
At Hexentanz there's the original article.