>>18624When you use your own router, you're putting a thick layer of security between your computer/devices and their modem.
It just sees packets coming in/out over the WAN interface. If you use their modem/router combo alone, that's allowing them a lot more access.
Let's just say Comcast is hypothetically evil, when using their modem/router combo alone, they would be able to map out your whole LAN, see all devices on it, and snoop on any unecrypted traffic that's being sent between devices on your LAN.
They would be able to preform countless (known/potential) exploits that can only be performed on other computers in a LAN.
This is all solved by simply using your own router, mine runs OpenWRT.