This may well be. Also a challence/response system, like previously
mentioned qsecretary, stops most of the junk.
They both have one thing in common: they're antisocial to the rest
of the internet. It is a bit like trowing your garbage over your
fence to your neighbours garden. Sure, you've got rid of it, but
created problems to others.
Both also have the same cost-shifting element than spam has: to get
rid of your junk, you shift part of the cost of your e-mail to
others. Greylisting increases senders mail queues unnecessarily, c/r
requires human intervention and more often than not the challence is
sent to innocent 3rd party.