Some search engines cannot index framed sites. Entry pages must be created for these search engines. Frames often slow the pace site loading. Multiple frames takes longer to load in a window than a single HTML page. Frames are hard for visitors to get rid of.
Frames always reduce the viewable space.
This is particularly important on smaller monitors where space is a premium. Unless implemented properly, framed-pages are very hard to bookmark. This can be a major headache to visitors wanting a specific page on your site. Virtually none of the large, professionally designed sites use frames. In some instances, a site designer would be better off using frames, especially for large, highly organized lists.