DigitalPoint forum thread named getting listed on Yahoo in 24 hours argues that by adding an RSS feed to your site and ensuring Yahoo knows about that feed, will in fact get you indexed more quickly. Of course this is disputed and there is no scientific proof of this as of yet.
In the context of tagging, RSS is an invaluable SEO tool. But outside of that, it does clearly help with site visibility. Black arrow is the Yahoo! Directory listing for this site, the blue arrow is the RSS listing for this site and the green arrow (box) is the My Web tagging I have done for this site. All of this increases the visibility of ones listing in the SERPs.