You're right. For average Joe there's usually no need for 4 PCI slots. However, taking me for example: in my 4 PCI slots I have an X-Fi sound card (no USB device comes anywhere near the quality/power), two raid controllers that I use for raid 3 and raid 5 (mobo doesn't support raid 3 and its raid 5 features are not sufficient enough for my needs) as well as a wi-fi card (yes there are USB alternatives but nothing with the same processing power).
What I'm trying to get at is that for some people 4+ PCI slots are needed as USB devices do not have the processing power to replace some hi-end PCI devices.
You have not one but two raid cards on a bandwidth limited PCI bus?