There are a lot of people running MythTV under Ubuntu, for example. If you're running a media server with the graphics card connected to a large format display (a projector, for example), and you can't get usable video output, then it makes the whole system worthless.
The same goes for people who use a PC or laptop as a multi-role device, for work and for personal use. I fall into that category. I'm trapped into using an X1400 because that's what was available in the laptop that I purchased, and I have a fixed long-term equipment investment in that. The fact that I can watch a video without a problem in Windows but it looks like garbage under Linux is very frustrating, especially when it's obvious that it's not the fault of the OS.
It makes the device useless for doing video editing, as well. What about those "workstation" customers? Or does "workstation" only mean 3D modeling in ATI's view?