if you have a tivo, it will be necesary, but only on occasion. if the box is not hooked up via phone, the box will not communicate with the tivo service, and after 30 days or so, you will start to get a nag screen about it. I have also read here that if you leave it too long, you loose the abillity to record.
About the electrical intereference, that may be an issue for houses with older wiring, but as I said before, my jack has worked flawlessly since march.
Also, if you get a wireless jack, try to make sure to get one that is certified for use with modems, those are more compatible for the sitution you have.