Blocking relays stops spams in progress. I've seen this happen often enough that I know it's what I have to do. But I've had two blackhole mirror sites drop off the list because they could not afford to block somebody that I had to block. But blocking relays doesn't stop the phenomena of spam, in fact it doesn't even slow it down. Consider the fact that I only blackhole when I am myself spammed.
Spammers do still send a lot of spam directly.
So I know that the blackhole list does some good. But it is not a fix to the underlying problem, and while I have no direct economic incentive to block spam, spammers perceive a very real and direct economic incentive to send it to all of us. So, yes, do sign up for the blackhole. If half the ISP's in the country would just refuse to exchange packets with most of AGIS's customers, maybe the other half would feel so much pain that they would come along for the ride. Who knows, perhaps we can isolate the spammers so they can only spam eachother.