Frames on a web site usually end up confusing web sites (and users sometimes). This happens because with frames there is really multiple pages per URL that the search engines can link to.
A search engine may crawl the content section of a website and like this content a lot, but when they link to it from their SERPs the user is brought only to this page and not the actual frameset that contains the navigation and other important elements.
With CSS, frames shouldn't really have any use anymore these days. Virtually all the positives for design that frames used to present can now be mimicked using CSS.