Not to discount how "frequent" users press the back button, or that it simply isn't a "good idea" to "break" it, I would offer a different suggestion: The back button should take the user to somewhere before the user got to where they are now. In a lot of cases it makes more sense and is more usable to not take them one link-click back (and may be much easier to implement). For example, take browsing a photo album. The user make one click to select the album, is presented with thumbnails. Another click on a thumbnail shows that picture with next / previous links. At this point the user navigates through the album. When they're finished, they click back. At this point, it's more convenient and intuitive to go back to the thumbnail than to the previous picture.
In short, the back button should do something, but exactly what it should do depends on the application.