One of Apple's core design philosophies is that when something is not in active use it should be hidden from view. Through this perspective it makes sense that they would hide scrollbars when the user isn't doing any scrolling.
That being said, I would rather have it always be visible because I'm not that much of a minimalist and I still think that non active information has value at a glance.
Edit: This doesn't mean Apple tries to hide everything that isn't in active use. It's a design convention, not a law.
I miss this behavior all the time when using Windows. I recently discovered that Windows actually has a setting to enable this behavior, but it's up to each individual application to check for the setting and implement the behavior. And as far as I can tell, literally no one does.
On a similar note, I get annoyed with how Excel won't show the highlighted row or cell when moving focus to another window. Sometimes I want to look at that specific row while using another program, and it's hard to find the data that was just highlighted when that disappears. I don't for the life of me understand what problem that behavior is supposed to help with.
That being said, I would rather have it always be visible because I'm not that much of a minimalist and I still think that non active information has value at a glance.
Edit: This doesn't mean Apple tries to hide everything that isn't in active use. It's a design convention, not a law.