Frames, or the combining of HTML pages in a single browser window, is a controversial subject for site designers. (Just to show where my bias lies, I'm in the scowl camp. It's not that I hate frames, just that I think most sites would be better without them.) But let's look at the pros and cons before you decide to add or not add frames to your site.

Pros

1. Ease of changing the navigation bar. Sites set up with a frames nav bar are incredibly easy to maintain. On a 200 page site, only one page needs changed for all pages to change.
2. Consistency. People know exactly where the nav bar is every time.
3. No scrolling to find the nav bar. It's always there.
4. Harder for visitors to leave your site (this is also a con). Site designers can make the HTML so that offsite links appear in the larger frame instead of taking the visitor off site. A good (or bad) example of this is The Mining Company which uses a small nav frame on every page of its site. All linked pages appear in the larger frame.
5. Advertising frames can always be seen.
6. Newer, embedded frames would be great for long lists of changing content if they were better supported by all browsers. They would also be perfect for embedding a nav bar frame in many pages.

1. Frames are not supported by all browsers. Admittedly, for most sites, users of older browsers represent less than 5% of their overall traffic.
2. Some search engines cannot index framed sites. Entry pages must be created for search engines.
3. Slows site loading. Multiple frames takes longer to load in a window than a single HTML page.
4. Harder for visitors to get rid of frames. This has raised some legal/moral issues because frames can give the impression that content of SiteB is owned by or originates from SiteA. Where things get nasty is if sites' A & B are competitors.
5. Reduces viewable space. This is particularly important on smaller monitors where space is a premium.
6. 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.
7. Unless implemented properly, it is possible to design a navigation frame which cannot be fully seen on all browsers/screen resolutions, making full site navigation impossible for some visitors.