While there are a handful of good uses for frames, most actual uses are unnecessary. Many people absolutely hate them. One reason for this is that frames are often misused, with excessive numbers of frames taking screen space away from the main content of a site in order to present unneccessary things like distracting animated ad banners. Thus, somebody who finds something interesting buried deeply within a framed site might have trouble getting back there later.
Navigation can be difficult and inconsistent.
In Netscape 2.0, the BACK button took you all the way past any frames you'd surfed through to the previous site before the frameset, making it really difficult to retrace your path through the site. A user with a lower-resolution screen than the designer expected may not have room for all of the frames' contents, and some of the graphical navigation bars may not fit. Sometimes a designer will make such a frame nonscrolling, so when it's too small to fit all the content the user has no way to get to the remainder of it. Since many search engine robots still don't index frames, a framed site may be cutting off most of its content from the indexers. And any indexer that does index frames could send users directly to a frame, and the user following such a link would have no way of finding its full frameset context which might have essential navigation controls not found in the frame itself.