First of all, a cable modem can do 10Mbits per second upstream, and mine can do around 45Mbits per second downstream.
Second, just because downloading the file from the internet to the modem is slow, doesn't mean you have to have it travel from the modem to the computer slowly. And, I only have a 10baseT NIC card, not a dual speed.
Third, USB internet gateway routers can have their own power source if you look for the right ones.
Basically, I'm talking about my computer only. I already had everything I needed. It all came with the modem. I'm now on USB and it works slightly faster than my 10baseT NIC card and I don't have to waste an IRQ and PCI slot on the NIC either. There really is no reason why I shouldn't use USB.