IMO that's mostly a window-management problem, not an app layout problem. The point of an ultra wide monitor setup (other than flight sims or something) is to be able to view a bunch of different things side-by-side.

Edit: speaking of which, now that we've come almost full-circle from no tab support, to multiple tabs in the same process, to one process per tab, it seems to me that tabs themselves ought to be part of the window decoration, not the app. I mean, they're useful for almost everything you might want to have multiples of (editors, file managers, terminals, etc.) so why force every app maker to implement them over and over again?

To be honest, it's not just for phones. The wider the monitor, the more I'd need to move my head if a website uses the whole space, instead of keeping it centered. Obviously it shouldn't be too slim but you can't really just fill an entire monitor or align your content to the left of the screen anymore nowadays.