Let's just admit everyone gets what they get not one view of the site, but some view that works pretty well for most people, and will look lousy to others running older or more esoteric browsers. The common response is yes. Mine is maybe. In either case, it's just impossible to offer that rare user the same page that almost everyone else gets. Detection or sniffer scripts are already used on the web.
Some site deliver pages especially made for certain screen resolutions and browsers.
This allows designers the control that they want over the placement of each and every pixel. Knowing screen resolution and browser brand and version to dish up the appropriate page can let designers go back to controlling font size with fewer complaints from critics. It allows them to set elements like tables to a specific width, and not the looser percentage of viewing area. But scripts are limited. You'll never see a script sniffing for Opera, or WebTV, or Mosaic, or Netscape3.0.