It's not a simple black-and-white issue with just two choices.
Sure, you can have a fixed-width design, or you can have one that scales proportional to the page, but there are other options as well, like
- constrained liquid layout, where you set a
max-width to stop the design expanding ad infinitum.
- semi-fixed layout, where some elements (eg navigation) have a fixed width, and the main content area expands to fill the space. May also be constrained as above.
- elastic layouts, that depend on the base size of the text (and may be constrained within the window or allowed to expand beyond it)
- responsive design using media queries, where a series of fixed width or liquid designs are made and the browser uses the one that is most appropriate to the window size.
Out of all those options, I would say that a single fixed width is the worst possible, and an unconstrained proportional layout is the next worst.
A single fixed width will be absolutely fine for some people, but will be wrong for others. If you have a large window, it doesn't make good use of that space ... but if you have a small screen, it will be too big, and that's definitely worse. Any site that generates a horizontal scrollbar in an 800px window is a failure, in my opinion, because it pays too little heed to users' interests, and puts the designer's aesthetics above basic usability.