The answer accepted by the new Alexandria is excellent. it goes in the details, so I do not have to do it. I'm just going to say that for these reasons, in general It's a very bad practice to hide the scroll bars. This is a very familiar feature, and the usual features meet the expectations of users. If you meet these expectations, you create confusion.
Even if Apple does it; there is a reason why they allow you to change this behavior in System Preferences »General. As slight as the confusion is, the confusion is always bad. And for the computer illiterate, the confusion can be greater than we think.
Of course, all of this assumes that you are not creating some kind of poetic software and that you want to arouse those particular emotions. I can not think of an example.
But the bottom line Is it depends. It depends on your application, your users, and the circumstances surrounding them. Even for me, it's annoying to have to scroll the screen to make the bars visible before you can drag them, but I'm using a magic trackpad, so I like them hidden (so that depends on the ### 39; user).
Pentadactyl for Firefox is a good example. It's an add-on that gives you Vim keyboard shortcuts and a Vim-like status bar, and lets you do it all with your keyboard. You scroll with
kand the status bar indicates the scroll position. Now, it's suddenly logical to hide the scrollbars.
Here is a screenshot. On the right side of the status bar, it is written "Top". (The picture comes from a blog post I wrote about it – original):
Once you have scrolled, it displays your percentage position. Here is a second screenshot, taken immediately (original):
Note, however, that scrollbars are still useful and visible in areas such as
textareas, like the one I'm typing. Here they are always expected. It all depends on the situation and circumstances.
Every year, my father becomes more and more efficient with his Mac. Still, he tends to drag the bars rather than "touching the scroll" with his magic mouse, and always uses the previous and next buttons. However, his mouse at work has a physical scroll wheel, and that one he uses. Perhaps because its purpose is more obvious – more familiar, no confusion.
Is hardware and software design evolving too fast and leaving some users with low literacy skills or is it a natural and necessary rhythm to which they must choose to keep pace? Well, I guess it depends on your target audience.
Warning: I am not affiliated with Pentadactyl. He and Vim are just good examples.