London Underground has a Tube map that I want to reference on my site, however it is buried within a frameset and I can only point to the top level and hope people will be able to locate it. This is unsatisfactory. I could be examining the code find the real URL of the map and link to it. Frames make it much more difficult for readers to establish consistent, reliable expectations about the behaviour of the constituent pages. Bookmarking and the History list become unreliable as these references do not include a representation of the state of the frames on the page.
Frames usually have an invisible active window.
Other frames become active when the mouse is clicked on it, difficult to do without a mouse. Screen readers tend to treat each frame as a separate window. This makes following the content and navigation difficult, as switching between the active frame is awkward. Frames inherently steal real estate and cause problems for small monitors and low resolutions. If people are using a large font they may be required to scroll horizontally to read each line of text, something that won't endear them to your site.