Unfortunately, I think this is what might be called a “misfire”. The main problem is that in hiding the thumb of the scroll bar by default, you are immediately up against Fitts’s Law because the reduced size of the target will slow its acquisition. That’s an HCI fail – and one that will ensure you’re never going to work for Bruce Tognazzi.
The other problem I have with it, from a methodological point of view, is that Giordano is taking his cue for the design from current, mostly mobile, touch interfaces. These sometimes exhibit similar scroll bars in order to reduce clutter on the screen. Clutter is of course a good problem to solve for in the highly constricted world of mobile and tablet UI. But desktop interfaces are a completely different kettle of fish. For starters, the vast majority of people running Ubuntu will be geeks with high resolution screens with oodles of real-estate available. Indeed, even if they’re not geeks, it’s hard to find anyone with a screen of less than 19″ at 1280×1024 these days. So that’s a UCD fail in not considering your users. The aforementioned Fitts’s Law issue is also aggravated by large screens with high resolutions because of the large distances between pointers and targets.
So it’s a nice try. But no cigar. I’m turning them off, and so should Ubuntu, I’m afraid.