Scroll bars have always been both control (press here to move around) and indicator (how much of this document is there, and how far through it am I?).

Always-on-screen scrollbars are only useless if you think that indication is useless. Many people find it quite valuable.

They are handy on a tablet. At the same time they are not since dynamic content might be loaded so the true size of the page is not shown to me by the scroll bar.

I restored them and made them fat again in FireFox. It was getting too thin and hard to find

I hate when magically appearing scrollbars cause the screen to redraw - I especially hate it when the content needs a scrollbar if NOT present, so it appears, which makes the content reflow so it doesn't need it, and it keeps swapping back and forth.

Good times.