Im only using one of em, for a Hollywood DVD card (can be a pain in the arse in XP though...). Ive got an nForce2 NF7-S so I dont need to use them for anything like sound, or NIC/Modem. So, thats 2/3 cleared.
I sometimes put my Realtek NIC in, if Im installing Linux and I dont wanna p*ss about with drivers and that kinda stuff. (im still a Linux n00b )
But, I suppose that DVD cards, and TV cards are cheaper to produce using PCI, rather than USB.
They will be phased out within the next 5 years, especially when boards like the nForce2 make such a good bargain, including NIC, decent sound, and RAID interface.
All in one solutions like the nForce2 are the future, not PCI card, after PCI card, after PCI card slotted into a Mobo.
Just my 2 pennies.