Continuing the discussion from In screenshot, Apple’s next desktop OS hides web addresses:
But it’s NOT useless when it’s not actively being used. First of all, it shows a novice user that there is a thing there to interact with; the novice user will likely try interacting with this GUI element and thereby discover what it does. This is called discoverability, and it is a design principle sorely lacking in modern UI’s (again, understandably on cramped screens, but not so much on big ones).
And secondly, even for a user who has passing familiarity with common UI design and knows what a scrollbar is, it provides useful visual information: viz, your position in the document you are currently looking at. (Hell, in Chrome it tells me even more; if I search for text, it marks every instance in the document where that text occurs.)
My point is, whether or not the tradeoff is worth it is situational. In this example, the gain is a few extra pixels in the viewing area and the loss is discoverability and secondary information about the content you’re viewing. I contend that this is a reasonable trade on a phone and not on a desktop.
Your point that OSX and iOS have not completely converged is fair (and Apple doesn’t seem nearly as determined to pursue this line of thinking to its ridiculous conclusion as Microsoft at the moment), but I content it’s a pretty big trend in current UI design. OSX, Windows, and Unity have all, to varying degrees, started to eschew categorized program launchers in favor of the Single Wall of Launchers popularized by iOS, and GNOME has completely separated its launcher, desktop switcher, and other basic UI components into an interface that is completely separate from what you see when you’re actually looking at and interacting with a program. (And here I thought the entire point of going with a GUI was to make interacting with your computer LESS like using vi, not more.) They’re all – again, to varying degrees – trying to get away from the desktop metaphor.
Your point seems to be “If I’m not using it, why does it need to be on my screen?” That’s a fair question. But here’s another one: “If it’s NOT on my screen, what the fuck else am I using that space for?” You say it’s not about saving pixels. Okay. Then what IS it about? What purpose does it serve?
And “it’s prettier” and “it’s cleaner” ARE perfectly reasonable answers; I won’t fault you for them. I’m merely saying that reducing the amount of information instantly available to an end user is inherently a tradeoff, that sometimes it’s a good idea and sometimes it’s not, and that those conditions may largely depend on screen size and interface device.