Spiders crawl text and feed search engines, the search engines index text based of key indexes. These key indexes are built off guess what! Keywords! Text is their bread and butter. This is where pages built heavily in text excel, where image laden and Flash only sites wither. The technology required to decompose visual images and audible sounds into text components does not exist widespread. You may have your entire product catalog, images, descriptions, prices and more in your bleeding-edge-state-of-the-art Flash presentation, but none of the search engines will be able to index any of that content worth a damn! Even by using ALT tags and other SEO tricks for non text content. Real Visible Text trumps any hidden text!