No advantage and some disadvantages.  Cable modems that connect directly to a USB port have disadvantages, and you'll have more flexibility with one that has an ethernet connector.  I've connected cable modems and DSL modems to a USB NIC when a PCI slot hasn't been available, but you can run into compatibility problems with USB NICs also - sometimes they'll disconnect for no apparent reason.  If you've got two PCI NICs running smoothly, I'd just keep it as it is.