What I get from buying a "Linux machine" isn't just avoiding purchasing a Windows license. (Those tend to be paid for by all the software bundling, anyway)

What I get is a computer that is tested on Linux and thus is known to have well-supported hardware.

I've had plenty of computers that "work" in Linux, but some of the smaller details (suspend/resume, advanced functions of the audio hardware, etc) would either not work, or would only work after considerable effort.

Installing Linux in favor of Windows won't make computers cheaper. Microsoft sells OEM distributors their Windows 7 Home license for around $50. Then, the reason off the shelf computers come bundled with so much bloatware from the likes of McAffee, Symantec, and 100 free hours of AOL is because those companies pay for it. In many cases, pc manufacturers actually return a net positive on installed software.

Geeks optimize everything. 99$ extra for Win (esp. if you don't use it) is 99$ wasted. Many of us are also finicky about taking a stand. A thousand geeks saying no to 99$ extra is a lot of money and a community in itself.

What would you use an Alienware Linux machine for?

There are many companies (and government agencies like NASA) that do a lot of high-end 3D visualization and processing on linux. Having one of these desktops would be nice in one of those environments.

The dollar sign goes before the number.

It's a tax paid to Microsoft. It's a matter of principle.