Sounds like a decent way to keep track of your search results... how often do you forget to bookmark a site that you stumbled on during your search before reaching the best site? Certainly, some of those other sites you saw had value and could be useful down the road.
I don't see this being manipulated for
SEO (yet), since it appears that everything is "within your own Google account". If they allow the ratings and/or the number of bookmarks to influence the rankings, you know that will be exploited ASAP by auto-account setup scripts, etc.
Google isn't dumb enough to get into that again are they?