I propose leaving the default site skin the way it is, but I'd like to see that we work consciously towards having a variety of site-skins where at least some of them are designed to fill a higher percentage of the screen width and some are designed to fill a lower percentage of the screen width.

As an accessibility issue we need to account for all sorts of needs - whether it's having a low percentage of white space on screen, having a narrow column to read, having enough space for magnification, light-on-dark, etc etc. Infinite Diversity in Infinite Combinations :)