Seems to me that the benefit of cutting down on Spam would be worth the trouble of using port 587...
Blocking port 25 is just a quick-n-dirty 'fix'.
What will happen when virus-writers are going to spam using 587 (The credentials are stored on the users PC anyway..)?
What would people do to stop blog-spamming? Blocking port 80 sounds like fun.
Spam will be there as long as you can make money with it.