Just noticed this while doing keyphrase research for one of my client. Repeated the search many times but got the interface just once. Also the buttons were for the sponsored listing only, not the organic listings. While there has been an option in Google to report bad listing or if you are not satisfied with search results, many people are unaware of this, as the option is placed in an obscure place ( 'Dissatisfied? Help us improve' link at the bottom of search results) and one rarely gets things corrected after reporting. Hopefully, this time Google is moving in the right direction. Humans are always better than bots in judging quality and by allowing people to directly vote yes/no for search results, Google could use the feedback to improve the search index dramatically. Your thoughts on this...
I think that you'll find that the 'was this link useful?' is used only for the AdWords listings, so isn't anything to do with refining the organic listings.