For every resource that goes unused is some resource gained. It is quite possible, that to achieve similar benchmarks to Windows 7, some corners needed to be cut (quite literally), in order to facilitate the implementation of the Start Screen and its live app update/RSS news feed-style components. This is just a guestimate, as specific information as to how and why some features were truncated altogether has not been made entirely clear.
The official reason for the desktop change was that Aero was designed to highlight the app currently functioning in the foreground, and that the new style does this more effectively. Ultimately, another reason comes to mind. It is not so easy to move around titlebars and windows that have excessive transparency on them and in them, perhaps by default. Thus, the new Windows desktop theme tries to fit in with the "Start Screen" look and feel to some degree.
There can be no doubt in my mind that additional rounded edges for every window, in a 3D accelerated environment, ultimately add up after awhile. So, too, does transparency. There was a time when windows7forums.com used almost exclusive transparency on most of its content, and many users on slower computers complained of bad load times; not because of any server or connection difficulty but because of browser rendering in relation to disk I/O and video processor capabilities. Unless we hear it from a dev team member, or straight out of the horse's mouth, we may never know why the transparency and rounded edges were truncated to such a degree,
However, I imagine that this has taken place to make room for new features, to make the desktop easier on touch screens, and to one day, in the distant future, make the Start Screen as good as the desktop, or vice versa. Judging by some things I saw in early alpha builds, like the user profile picture and options appearing on the bottom right hand corner near the system tray, I suspect much experimenting went on with improving and/or streamlining the desktop. It is quite possible the choice was made, when resources are constrained in project management, to streamline the desktop and focus more on the Start Screen.
This is all conjecture, of course, but for Windows 8 to succeed in the marketplace, it must be known as its own beast. Re-using the exact same theme from the prior version of Windows would likely have elicited more protest than tinkering here and there for some minor changes to the desktop behavior. After all, it is Microsoft that has mandated the development of "Metro" or "Modern" apps, and not a renewed emphasis on thin-client, cloud-based services, and not necessarily traditional desktop applications.
I am left to find myself using the desktop more and more in Windows 8; conscious of the Start Screen's existence, and using it when necessary, but not finding it or its related applications (even third party ones) appealing. Many of the third party apps I have encountered are simply thin-client API versions of websites that are easier to navigate on tablet computers than in a web browser. I think the only exception to this, for me, may be the Kindle app, which I have tried to make some use of.
Basically, an application programming interface (API) is set up by the service provider (example: NBC-TV, Amazon, or ChaCha.com), and this allows for left-to-right and menu-based use of a pre-existing website that would not normally be intuitive on touch screen monitors. I find this to be deeply regrettable, since I am unlikely to find myself using a touch screen system that runs any version of Windows 8 any time soon. This is simply due to the realities that exist here, unless some client I haven't met yet goes on a buying spree.
The traditional mouse, keyboard, and desktop continues to work just fine for me, and my use of a "Metro" or "Modern" app like "Weather" or "Calendar" is something rare. In either case, I can just navigate to a bookmarked website and make the viewing and editing decisions I would like. I am unlikely to be highly motivated into a Windows Phone or tablet, since I am already using Android's Ice Cream Sandwich and Jellybean (Samsung Galaxy S3 and Nexus 7). I find these devices to be suitable for my consumer needs, while my real work and entertainment still takes place in a desktop environment, where instant access to reliable, productivity applications is just as necessary as it was during the Developer Preview and prior.