They maybe try to complain to the admin-c of the sender domain, but would never identify the IP of the sender beeing the cause of the problem.
And thats why I think (but cannot proove with facts) that whois is only used by RIPE members, ISPs, sophisticated network admins or blacklists admins anyway.
Maybe somebody from RIPE NCC can count how often dynamic IPs are using whois compared to fixed networks ...
They do and they do it a lot. That's why since 2004 changed lines and other poossible contact points are suppressed in the default output. The problem with misdirected complaints was getting bigger than the spam problem itself. You can filter spam, but you can't easikly filter complaints. Or at least it's not very nice to filter them. Groet, MarcoH