I really don't see a reason that wider monitors can't utilize their width on individual posts.
The circle shows that the width for text is limited due to the scrollbar, and if you look at the size of the post, it likely shouldn't even need it.
The scroll bar is there because of the length of the link, not because of the text. The system apparently won't let a link drop across multiple lines. This should be less of an issue when they finally let us embed links in text, rather than having to just have the full URL hanging out there cluttering up the post.
The reason for the width limit on the text area is for readability. At a certain point, if you make the lines of test too long, it will actually cause more strain/fatigue on the reader.
Rough estimate is that the displayed text will never be more than 740 or 750 pixels wide, no matter how wide you make your browser window.
