"STATE is important!"
Even if a user never touches a scroll bar or scroll button, the persistent visibility of scroll bars is a necessary visual feedback indicator informing the user where he or she is in the document: "Am I half way down?" "Am I at the bottom?" "The top?" Even iOS offers such (very spartan) "lines" indicating where a user is in a document or window.
It is said that Apple uses a focus group of one: Steve Jobs.
I appreciate his efforts at UI coherency and a clutter-free appearance, so long as they don't detract from the Mac's traditional "You learn by using -- not reading a thick manual," and feature discovery
via the user just using
But how does a novice aim at a scroll bar that is invisible? How does a user aim at a Safari Close Tab button that is invisible? How does a user aim at QuickTime Player controls that are invisible?
Not us Power Users, but average Mac users need to know what they're looking for, what they're aiming for. If that comes at the expense of a little clutter (as a necessary evil) then it should be done in the users' interests.
Aesthetics are an important priority, until, that is, when form overtakes function.