Why using (cat5) cable?
Wireless is very nice, but out of reach within the target costs per node. It would require a transceiver and a 230V power supply per node. Cat5 cable is the cheapest cable on the market today (even cheaper than simple twisted wire). Powerline communication is possible to but also rather expensive and requires operations in the meter cupboard when you have more power groups. Not really a do-it-yourself operation. Furthermore, on some places where you want to switch or detect something there is no 230V nearby. For example at curtains, window break detection or at door open/close detection. So in this case you need a cable anyway.
