I used to have a lot of expansion cards in my older PCs, but now I hardly ever use any. I have my video card in my x16 slot, and right now I have a PCI slot filled with a wifi adapter, which I could (and probably will) remove. Just there in case I need to tether to my phone in case of internet outage...though I can also tether by USB, even this card is not needed (harvested from my old HTPC).
I don't own any PCIe cards that aren't video cards, and I never have. The only thing I can see adding in the future is perhaps one for SATA expansion, since I've currently filled all my board's SATA ports, though I have a PCI IDE/SATA card that I could use if I needed one extra port...and I'd just move my optical drive to that one since speed wouldn't be an issue.