It depends on the page content.
If the content is volume-based (lots of images for example) then making the most use of the whole screen could be a good thing as, arguably, user will be able to see more items at any one time. (Although this approach may also run into 'Choice Paralysis'.)
However, if the content is mainly text (like Wikipedia) then we have to think about readability. Optimum reading line length is somewhere between 45 and 75 characters - somewhere between 7 and 14 words per line (http://baymard.com/blog/line-length-readability). If the lines are longer they become more difficult to read as the end of the first line moves further away from the start of the next line - The reader finds it more difficult to move smoothly from one line to the next.
A site may choose to use a narrower layout to avoid this.
As for scrolling, most users expect it and don't even notice that they're doing it (http://uxmyths.com/post/654047943/myth-people-dont-scroll). (I'm sure I've seen a study that even said that younger users actively leave pages that don't scroll... but I can't find that right now.)