I'm concerned about this, too. There have been times when a user's email gets thrown into Held Mail simply because I chose to remove a full domain from my whitelist, or their email changed slightly (e.g. from my-friend[at]mail.friendlyISP.com to my-friend[at]friendlyISP.com, or even the rarer change of top-level domains), or they simply moved to another ISP.
There are two big advantages to the Spamcop system over others like Earthlink:
Nothing gets lost without my knowing it;
I get to do something about spammers, beyond simply ignoring them.
I fear greylisting removes much of that.
Nonetheless, thanks for offering it, and thanks for making it optional