It hurts usability in several ways. For example, you can’t click on an iOS scrollbar to jump to a specific part of the document. Nor can you use it as an instant reference to determine how long the document is.
How would you propose doing that on a 3.5" screen without a mouse or stylus?
> Nor can you use it as an instant reference to determine how long the document is.
You scroll the screen a tiny bit and the scrollbar pops up. Again, keep the size of the screen in mind—making the scrollbar persistent would be a poor use of space.
I know it’s still there - that’s why I said “instant”. It’s a very minor concern, I’ll grant you.