OP is saying that the coax is already run through the house, so why run cat 5 ethernet everywhere to use a wired router.
From my point of view, the cable modem uplink is so much slower either Cat 5 or wifi that I don't see the point of favoring either of them over the other. The exception would be internal LAN connections, but I don't think multiple modems fixes that.
To answer the OP's question, the answer is that every new cable modem needs its own real IP address, and IP addresses are a scarce resource. This is why NAT boxes were invented!
Re: Why do you need 2 cable modems?
From my point of view, the cable modem uplink is so much slower either Cat 5 or wifi that I don't see the point of favoring either of them over the other. The exception would be internal LAN connections, but I don't think multiple modems fixes that.
To answer the OP's question, the answer is that every new cable modem needs its own real IP address, and IP addresses are a scarce resource. This is why NAT boxes were invented!