If an ISP gets a spam report, that's one thing. But, if the ISP gets a report that gives detailed information on just how long the spammer has been at it, and how they've been kicked from ISP to ISP, that's quite another. It may be enough to convince some ISPs to dump the spammer, since he has been lied to. This can make it hard to recall exactly who said what to whom and when. The power of DejeNews lets you network with anti-spammers who are speaking to you, as it were, from the past.
Spammers often change little bits and pieces of their spam as they fine-tune it.
If they find that they've left themselves vulnerable, they change the ugly bit and continue on. They hope that nobody realizes that they've fixed whatever it was that gave them away in the first place. However, DejaNews is the answer here. By comparing past and present spams from the same spammer, one can find interesting things which can finish a puzzle sometimes. This is not frequent, but it does happen.