The idea behind the green button is to fit the window to the size of the content. Mac purists (ie. people who aren't used to Windows) would say it's the best way of doing things since you don't have a window bigger than you need it.
Of course this is useful if you're on a large displays - you don't need a full screen window for the Google homepage
Saying that, as said before, it's incredibly inconsistent. Even using it in Safari annoyed me since some pages (like this forum for example) re-arrange themselves to make best use of your window size. Widescreen was invented for a reason