Imagine you are a painter and a salesman tells you that you should get rid of your tools in order to have more space in your studio. "Your workspace is not cluttered by all the other things like brushes and colors. What matters is the painting, not the tools!"
This seems silly but it is analogue to what is popular in GUI design nowadays. Freeing the space for what you want to see or create seems reasonable. But the goal of somebody is not to just see something but to see what is interesting and to create great stuff. In offering the functionality to do that a visible GUI is superior in terms of efficiency and learnability to many alternative ways like hidden elements, edit modes or gestures.
But with the rise of the touch driven mobile devices the design decisions that were made for these devices are spreading. And screenspace is very, very scarce there, and it was tried to eliminate as much GUI as possible. But since mobile touchscreen devices and applications have the reputation to be instantly easy to use we apply their designs to other domains. On touchscreens – regardless of their size – we invent complex multitouch gestures and on big displays we get rid of scrollbars
But they ground on a conception of a GUI that merely takes away space for triggering actions that can be accomplished with hiding them via modes or using gestures instead. What should software do is to enable us to reach our goals. We want to read the right stuff and we want to create content efficient and without learning about gestures and hiding places of functions. The conventional select-command GUI paradigm does a pretty good job in doing so, even in contrast to other established techniques. But whether you use the trusty old WIMP paradigm or alternative approaches: don't hide the tools!
The unity design team came up with a solution for scrollbars that does save screen real estate and offers visibility. I strongly recommend to have a look at their design!