It's effective in getting these companies to clear your data. But do you actually get fewer unsolicited calls/emails, etc.?

The do-not-call has been fantastically ineffective for me. I imagine this could be the same. It's also possible that all of the companies strictly adhering to the FTC rules (i.e. those on the OP site) constitute a minority of the market, and spammers are getting data from other less scrupulous sources.