When Apple released Mac OS X 10.0, the operating system was virtually unusable, and not at all ready for prime time. Since Apple realised it would take them a few releases before Mac OS X was ready for general use (I'd say they hit it at 10.3), and because it would take developers a while before they had adapted to the new developer tools and paradigms, Apple included the Classic Environment.
Classic was a software layer which used a MacOS 9 system folder and a New World ROM to run OS9 applications within Mac OS X. I'll admit I'm not entirely familiar with exactly how it worked on a technical level, but it felt very similar to the 'coherence' features that would pop up in tools like Parallels and VMware years later. Classic met its maker when Apple shifted to Intel.
The desktop in Windows 8 serves the same function. Microsoft knows Metro isn't ready yet for general use, and that developers will need time to adapt to Metro and the WinRT programming model. In addition, it's clear Metro is still in flux, with features still being added and, in my view, a general lack of 'compatibility' with larger displays. Heck, it's not entirely clear how large, complex applications are going to work in Metro (Office, Photoshop, and so on).
The desktop is Microsoft's temporary answer, much like Classic was for Apple. It allows users to run 'classic' applications in a way that, despite the jarring Metro-to-desktop switch, is still more seamless than Classic in Mac OS X (which required an entire boot-up of OS9 before applications could be launched). Until Metro has the features - in whatever forms - and applications users need, the desktop will be there as a fall-back.
As John Gruber notes in a reply to my tweet, a lot will depend on if users will actually demand native (i.e., Metro) applications. In other words, will they like Metro, and thus demand Metro applications, or will they hate it, and stick with the classic desktop as much as possible?
For Microsoft, this is the perfect moment to attempt such a massive transition to a wholly new environment. Windows 7 is a fantastic operating system, and will be able to serve the more conservative parts of the market for years and years to come. Your business not ready for Windows 8? Here, have a Windows 7 license instead.
A lot is riding on Metro in Windows 8. Millions and millions of users will be confronted with it come late 2012, whether they want to or not. And, if they actually like it, it will, quite possibly, lead to more Windows Phone sales as well. Interestingly enough, this is a variation of how Windows became popular in the first place: people wanted to use the same software at home as they were using at work.
Redmond is betting the company on Metro. I'm incredibly curious to find out if the gamble pays off. Coincidentally, I haven't had the need to boot back into Windows 7 on my laptop just yet.