Why You Shouldn't Use Frames
Frames are a really nice idea—but they don't work very well.
Google summarizes the problems of frames well. This is excerpted from Google's Guidelines for Webmasters.
Reasons your site may not be included:There are
...Your page uses frames. Google supports frames to the extent that it can. Frames tend to cause problems with search engines, bookmarks, emailing links and so on, because frames don't fit the conceptual model of the web (every page corresponds to a single URL). If a user's query matches the site as a whole, Google returns the frame set. If a user's query matches an individual page on the site, Google returns that page. That individual page is not displayed in a frame -- because there may be no frame set corresponding to that page.
If you are concerned with the description of your site as seen by search engines, read "Search Engines and Frames" It describes the use of the 'NoFrames' tag, which is used to provide alternative content. If, instead of providing alternative content, you use wording such as "This site requires the use of frames" or "Upgrade your browser", then you are excluding both search engines and people who use browsers with frames turned off. (For example, audio web browsers, such as those used in automobiles and by the visually impaired, typically do not deal with frames, which are a visual mechanism.) You can read about NoFrames in the HTML standard. (emphasis added)