Rdio is I think an example of the aesthetic taken too far; when I first got it in an update, I thought it might have been a bug. It looks not drawn enough, like the product of a UI framework that deferred look-feel and died due to an uncaught exception.
A lot of UI styling is just cosmetic, but some of it is there to create affordances.
When I first saw the UI I thought there was a stylesheet missing or something. After several force reloads it started to dawn on me that they were serious.
I am generally very pro-minimalism and hate skeuo design but this is not only taking minimalism too far but failing to understand that a blinding white screen is visually exhausting. There's a reason almost all "pro" apps have a muted, grayscale palette.
Their designers stripped away everything that was unnecessary, and allowed the content to become the design. It's a very difficult aesthetic to pull off because it relies entirely on the most challenging aspects of UI design: spacing, grids, and typography.
At first, I didn't like it either, but over time have come to love it (and have become inspired by it in my own designs).
Alas, it is possible to strip too much and I think Google is guilty of that.
> aspects of UI design: spacing, grids, and typography
I'd say these are more aspects of content design, albeit still very important in UI, but less so, for a simple reason: typically there is less content.
You still have to have some visual hierarchy, make important things prominent and less important less so, mark interactive elements as such. When you strip too much you are forced to use whatever left for that and it can be a stretch, like excessive use of white space.
A lot of UI styling is just cosmetic, but some of it is there to create affordances.