You also need to consider how the user would arrive at your site in its framed state. Are they leaving another website and the site they are leaving them frames your site with the option to come back? Its certainly ethically wrong, but I see very small reasons why it could in very rare circumstances be a good thing for the end user.
If we think in terms of Google cached pages. Google use the top section to provide details of the page etc etc. Thats one example where it may be useful.
If the site in question is not in any way doing you favors in terms of traffic, it might be worth looking into blocking the referring url?