My browser window is nearly always about 700px wide – that’s half of my screen. Two-finger scrolling makes vertical scrollbars just as easy to use as horizontal scrollbars but 1000px wide designs are not optimized for 700px wide windows [+].
I certainly don‘t want websites to start using 700px wide layouts. I would, however, love to see more websites that change their layouts depending on the width of the browser. Twitter could drop one column if needed. Many browsers already make such changes easy and it is just as easy to force the common denominator layout on older browsers. That would be completely in line with progressive enhancement.
[+] To all the web designers out there: I can understand that it is not exactly your priority to make it easy for someone who like 700px wide browser windows to use the website you are designing. But could you please at least test whether everything still works even when the browser window is that narrow? I really should always be able to scroll to any button and backgrounds shouldn’t disappear.
While developing alternative display modes for non-standard resolutions is certainly possible, it unquestionably increases the time and expense of design, development and testing for what is arguably a negligible gains in terms of the percentage of users you're serving.
It‘s just that many, many websites are being optimized for low resolutions on mobile devices anyway. It would be great if that work could be repurposed for the desktop.