I would go even further and say that the lack of scroll bar in OS X has lead to lower uptake of read material.

Here is what scroll bar does for me:

1. It gives me instant feedback about the size of the document (the shorter the scrollbar the bigger the document).

2. It shows me rough position in the document.

3. Provides reading context (I'm 35% into the document and I'm reading on subject X) which aids memorizing the material.

I'm a mathematician and numbers are my thing. Some people use number memorizing technique where they walk through a familiar place and assign numbers to landmarks. And to recall numbers they walk the route again in their head and "read" the numbers off the landmarks. This is the technique those freaks that can recite thousands and thousand of decimals of Pi use.

In effect the scroll bar does the same for me. It's my landmark and I assign topics to its position which allows me to remember better. If I want to quickly re-read something weeks later I can open the document and within seconds get there just by scroll bar position. And one could say search would work here too, but often times it does not because I don't remember exactly the string I want to find in the document (and some documents are not searchable).

Once I upgraded to Mountain Lion from Snow Leopard, I found the default scroll bar setting extremely bothersome and my read content retention much much lower until I put the scroll bars back.

So another anecdotal reason why dynamically showing/hiding UI elements is usually a bad idea.