They said the same about greylisting. That the spammers would just write a retry logic into their sending daemons. This was several years ago, back when I enabled it on my mail server, and I am yet to see a single spam that didn't come from Hotmail or Gmail.
Sadly, greylisting is an absolute nightmare for server-admins on the sending end. We sent out time-sensitive emails to our clients and Yahoo's Greylisting could sometimes delay accepting the mail for upto an hour. This causes extreme frustration for our customers and some even leave as a result. Its even worse because customers are never aware of Greylisting and even if they do, Yahoo wouldn't let them have control of it. Asking someone to change their main mail provider doesn't end well either. This could also happen for confirmation emails when they first sign-up to your site. Spam does indeed cost a lot to businesses directly or indirectly.
