I'm not sure what you're suggesting, but I typically use a, uh, wide stance with browsers. As per my screenshot.
Some sites understand that and they're all like, hey, cool, we'll take up as much space as you want to give us. Not many, to be fair, but some. Must be all those usability studies you mentioned. But fully dynamic width is one of oh, maybe, two things I miss about vBulletin. I've adjusted just fine to the fixed width, but it's still weird to me that Discourse is more than happy to break its own rule with the preview pane. Suddenly, it's all like, hey, cool, we'll take up as much space as you want to give us!