I think for most web apps it doesn't make sense to allow the width to get so wide, except when the content being displayed is a columnar list and even then it's a pretty marginal benefit.

What I've done is limit the max-width to some amount of px/chars and allowed the remaining space be empty, with an exception for when displaying tables. Even with tables, the bigger width is only beneficial if either the contents of the columns are large enough, or there are very many columns to show. The solution in my mind is limiting the column widths to the longest content.

You could go columns for the content, but I think my ideal layout would still have the main content in a single column. I would put all of the chrome horizontally through. For example no header before and footer afterwards, put everything in different columns. Maybe even throw some extra navigation on the screen.

You don't need to use every pixel, just avoid putting things offscreen unnecessarily.

This seems really cool for tiling windows managers (even Windows has tiling options, although I'm not familiar with those). That being said, I still prefer a multimonitor setup on my tiling WM of choice.