Simply put, there's no decent software that I want to use that will run on a Mac.
I guess you don't do any commercial graphics, publishing, or media work!
Well, that's pretty well founded.
For audio, the answer is NO to a Mac. Remember what the question was... Desktop.
ProTools is THE software and the platform of choice for it is Mac. But that's in a studio, and not for a desktop computer. I'm not an audio engineer.
For a desktop computer, there are much better solutions that don't cost $10k+. Samplitude or SAW Studio are excellent. If you have MASSIVE requirements, then even in a studio you can't use a Mac for audio (as far as I know). You have to use SAW Studio because it is the only one out there that can slave other computers in clusters. That's Windows only.
Graphics? I know a lot of designers that use Windows for Photoshop and Illustrator. But most use Macs. The platform doesn't really matter a lot there and is more of a personal choice. Graphics are just about all on a desktop computer, so there's no big deal there. I'm not a designer.
Video... Hmmm... I'm not too sure what people are using now. Liquid used to be popular. But I'm a bit out of touch with the professional video editing stuff right now. It's been a few years. I don't know if there are video solutions for Mac clusters. I would guess that there are.
But for doing video on a desktop? Not sure. Final Cut is excellent. (Mac)
Audio and video can now be done on a desktop computer fairly reasonably in a commercial setting.
Publishing... Quark. On a Mac. (Last time I was working with print layout anyways -- it's been a few years.)
My audio and video needs are pretty simple. I use Samplitude and Sonar for audio. Video -- I only really do screencasts with Camtasia. Layout? I don't do layout for print so I don't use Quark.
This really goes to the difference between amateur, prosumer, and professional needs. My professional needs are most important to me. That's Visual Studio and a solid text editor. I use Photoshop (7) for graphic work as nothing compares to it. But my needs there are very simple. I don't need CS#. Samplitude is simply a stellar program and doesn't run on a Mac. I don't need ProTools.
Macs are extremely expensive. Not just the computer, but the software as well. There are just too many very good programs for Windows that are reasonably priced. There's just no way I could justify throwing out a perfectly good solution that I'm comfortable with for something that's 10x the price for a Mac. The computer isn't the real cost. The real costs are the programs that you put on the computer and the learning curve for them.
Similarly, it would be nuts to get an audio engineer to switch to a PC if they've been using a Mac. etc.
Blah... I need to get back to my beer then go to sleep!