A few years ago I worked on a digital library system and we would sweat over including proper metadata about articles in meta tags (derived from the Dublin Core attributes entered by the authors). That was until we met with someone at Google who worked on Scholar. He said there was no harm in including that metadata, but they usually had more success inferring things like authors, title, etc from the HTML content of the page so they ignored it for the purposes of indexing. Things may have changed, but basically I think you're right that semantic markup is easy to game so probably doesn't affect SEO. Doesn't mean it's not worth doing though :)