I also received this response from jkintner :
"I was just reading your cable modem wiring FAQ and bumped into this question. I'm a contractor in Las Vegas, NV. We actually require the termination of any open port or wall jack in the home because of leakage and ingress problems. Having an open port acts just like an antenna and ends up backfeeding signal back into the system.
One noted case was a customer having problems with their modem dropping off during the day time. We re-wired his house from tap to all outlets to try and fix the problem. After about 4 weeks of troubleshooting and getting an engineer involved, we found that a neighbor had an open port in a bedroom and was using a remote controlled car. The motor in the car was putting off just enough interference on the upstream carrier for the modem, that it was knocking our customer offline."
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