shotgunefx beat me to it.. the problem is that most of the time, spammers don't even care about what comes back. They fire off a few million mails and simply don't care at all about how many of them get rejected, bounced, filtered or otherwise don't reach their destination because if even 0.01% of recipients do react to these mails, they've at least broken even, if not made a profit. It simply doesn't matter whether you bounce a spam mail, sendmail rejects it, or whatever. The only way to combat theft of resources is to silently filter spam as close to the source as possible.

.procmailrc is just available to anyone, while the system's sendmail configuration may not be. If you do have access to it, all the better.

Makeshifts last the longest.