This decision frees developers from maintaining multiple code-bases, and can also be justified on the grounds that users are familiar with mobile devices. Never mind that users may be familiar with mobile desktops, but not actually like them. Never mind that mobile desktops are used for relatively trivial activities compared to a laptop or a workstation.
Just as on the Windows 8 preview, the preferred design principles in most of the free desktop are those used in mobile devices. Only KDE has resisted this tendency, offering a variety of views that share a common code base aside from their interfaces.
Under these conditions, a popular release of Metro might easily encourage imitators on the Linux desktop. Instead of innovating for themselves, free desktops developers could be distracted by a race to the bottom, with each development team looking to find a lower common denominator than all the others.
Should that happen, then the user revolts that are already happening might seem like minor grumbling in comparison. Everything that people already complain about -- inconvenience, restricted work flow, lack of customization -- would become twenty times more troublesome than it already is.
That is why Metro is potentially so alarming. If it is successful, the result could be a magnification of all the existing problems with some of the leading Linux desktops. The old values of the free desktop -- control and autonomy -- could be overthrown for inferior interfaces that in the end might become self-defeating. After all, why switch from Windows 8 to Unity or GNOME if all you are offered is more of the same?
The future may not be the way I describe. Metro could morph out of all recognition, or face its own user revolt. Free desktop developers could change their direction in the next year, or even find the failure of Windows 8 an occasion for self-contemplation.
However, given Microsoft's power to impose whatever it chooses upon users, the possibility remains strong that Metro could become the new standard for interface design on all desktops -- not because it is useful, but simply because it is what users are accustomed to seeing.
The fact that a standard Windows desktop is available behind Metro is some consolation, but not much. Microsoft is promoting Metro heavily, and whether that standard Windows desktop will be available in the final release of Windows 8, or disappear after a few minor updates, is uncertain.
Ordinarily, I don't care much about what is happening on Windows, and neither do many Linux users. However, in the case of Metro, I make an exception, and so should you. It just might be the first indication of a nightmarish future -- the return of everything we thought we'd escaped by switching to GNU/Linux.