PCI slots aren't just for graphics cards. You can also put sound cards, optical channels and all kinds of stuff in them.
Dual Displays are FAR more than just a luxury. I have a dual monitor setup in my studio, and it's so much faster and more productive than a single monitor, you'd have no idea unless you could actually try it out. I'd say it cuts the time it makes to put a song together for a band etc... down by about 60%.
I need tonnes of storage. Some of the audio projects I work on can take up many, MANY gigabytes. The largest project I worked on took up 186 gb, for ONE song (although it was about 35 minutes long). Live, for example, if you're streaming 26 channels of audio through Logic for 2 hours or so (which I will be doing in a few weeks), you can imagine that you'd end up with a pretty huge amount of data. Video editors have the same issue. Open QuickTime Pro, and record a few minutes of Uncompressed video with an iSight. At 640x480, that'll end up being well over 5 gb. Imagine producing a 20 minute short, in High Definition with uncompressed video?
I don't actually have a Mac Pro, I have a Dell in the studio (although I desperately need to change it to a MP, I made the biggest mistake of my life buying a PC for music work). Of that, three of the five slots are taken up. Sound Card is one, Graphics Card is another and then the wireless adapter.
The Dell only has an 80 gb internal drive, I use externals for all my projects. If I had a Mac Pro, I'd be able to have all my projects stored internally, but with easily exchangeable drives (when I fill them up).