Thanks to the Spam button, any Gmail user can quickly report new SPAM to Google.
Google can collect reports from thousand of users and maintains a list of spamming sources and deny new emails from them.
This list is nothing more than a RBL.
If Google had the good Idea to give free access to other users and even allow other RBL providers to merge the list with their own list, any spammer would think twice before to send SPAMs to Gmail users !
How spammer could take the risk to be instantly blacklisted by one of the most used blacklist with not luck
to reach any Gmail users, or so few.
Gmail addresses would become a last choice for spammers !