Hello, bjraines. You asked why people should want a fluid design. Long time ago, in the dark ages of the Internet, all designs were fluid because the Internet was meant as a medium to transmit content and not design. Original HTML had no previsions to set things such as the width of the page, because it was supposed it was the task of the browser to accommodate contents to whatever equipment the user should have. Now, it is 2006 and things have changed a lot. Graphic designers entered and pages began to fill with colors and images, and now every page should be an artwork. It is common the saying that "content is king," but at the same time everybody expects to get a website that can be hanged on the wall. You must decide what you want: if you want an artwork, then used fixed design. But, as Will.Spencer said, using a fluid design nowadays is not as easy as it used to be. Cheers.
It comes down to how good you are at designing fluid designs if you carnt design them then yeah, fluid equals ugly, but then if you can design fluid designs you come away with a site that looks good in all screen res and makes the most of all the space available intead of having wasted space on each side of your site. I say do what works best for you, but dont be scared to try new things and dont judge them to early some things take a wile to master.
Fluid designs are a lot easier if you don't have to deal with a lot of non-fluid elements -- like AdSense ad blocks.
I've always gone for 'fluid' design as it is known today. And does anyone ever track the screen resolution of their visitor so they can design from a place of knowlege? For one of my sites over 75% of my users have their screen resolution set to 1280x1024 or 1024x768. There were less than 5% are using 800 x 600 screen resolution. With regards to browsers its almost equally split 50/50 between IE and FireFox.
The whole design for 15", 17", 19" monitor debate. It's never ending. I design for 780 wide and centered most the time. Its simple and easy. Plus I am not very good at fluid design. And I dont mind the white space. Even on my 21". The average surfer is used to seeing many 750-800px wide sites, so they don't really care, for the most part. Even though more people are getting larger monitors (17's are now the standard for most store bought machines) some people, particularily those over 55 years old (like my parents) resize the monitor setting to a lower resolution so they can read 8pt to 10pt fonts.