This is where we put a detailed description of the company with lots of word tracked keywords. All popular search engines either use META tags (less and less) or, in the event of frames, they look for the no-frames content. So this is where we put all of the necessary keywords for the search engine to tag.
In addition, this is where we put a site map with all the html URLs listed out. This allows the search engines to crawl to all of the important pages and list each and every one of them.
If you're worried about the search engine finding frames with no content, like the wings or the bottom frame, don't be. First off, in each of those, we put a hidden link back to the index, so if the search engine happens across one of them, it is redirected back to our site map. Second, every sub framed page has a simple javascript include that redirects them back to index.html. So even if they did happen to stumble across a side html file or something, it will automatically redirect them to index.html
Most hard core coders still hate frames. I admit I was one of them back in the day. But while working for Virgin I had to do a lot of high end stuff and there really was no way around it, so I made the best out of the situation I could, which led to the aforementioned method that still works with search crawlers. The only cons are: 1) Netscape browsers usually are a little off here and there, you'll see some space near a frame. I usually say fuck netscape if it's a high end site anyway, but I can also work around the differences and make it different, yet appealable. 2) you can only bookmark the main index page, not any sub pages, but since they want flash anyway, bookmarking is limited to begin with.
The pros: much cooler look, but most important of all, that's the only way to get multiple flash documents to play on one page. Flash won't play right if two files are referenced on one html page!