The scroll bar is hidden on MacBook if the user doesn't move the mouse for some time.
This pattern is also used in other devices too, for example Android devices.
The scroll bar does not take much place but it informs a user of where on page he is.
Is it a good approach hiding the scroll bar when the user is presumed viewing the content?
On computers, I would argue no. There is plenty of space most of the time and it is not necessary. The extra 20 pixels is not going to inconvenience most people.
On phones it may be a different story. A lot of the newer Android phones have huge screens but a lot of people still own the much smaller devices. Those 20 pixels could make a huge difference if you're scrolling down an article of text.
It depends from user to user how they feel about it..it's look good to me as it gives a more cleaner view..
Additionally, you can give users an option where they can choose to disable auto-hide feature..
