This is not specific to X10, though I've seen some modules do this as well. Why do some PCB's have extra components that are not populated?
I have a bug zapper racket thing from Harbor Freight. It stopped working even though the batteries are good. So I did what any engineer would...I opened it up to take a look. I don't care that it's broken really, it was just a toy to kill flies when grilling. What would like to understand is why design a PCB with extra components that don't get populated? Is it general practice for testing the board? Is it a way to make production cheaper? If I add the components back in will I kill a bunch more flies? Maybe it adds an Easter egg that will enable god mode.