That's really the question at hand here -- whether or not there's any benefit to continuing the "never ending arms race" game. Some people think there is. Others question whether anything is really being accomplished. Certainly we're playing it out like an arms race -- ISPs block something, spammers find a new way to inject spam, and so on. The end result of lots of time spend on blocking thins, less functionality for customers ... but no decrease in spam.
I take it you've never run a mail system other than perhaps a tiny one for your friends. The alternative to the arms race is to abandon e-mail altogether.
The theory behind closing open relays, blocking port 25, etc., seems to be: (a) That will make it harder on spammers, and that will reduce spam -- some of the spammers will find other other ways to inject spam, but some will just stop, OR (b) Eventually, we'll find technical solutions to *all* the ways spam is injected, and then there will be no more spam.
Interesting guesses, but wrong. R's, John