One of the most appealing aspects of using X10 is the ability to dim the lights. Relay modules do not have that function.
If WS467(1) was moved to the other side of WS467(2), then they would not have to interact with each other. The actual implementation of this would not be easy though as it would require running wires to/from the switch that is already installed.
The main reason I do not like the in line modules (XPDF, XPFM) is that the light switch would have to be left on in order for them to be able to control the lights. Although my kids do enjoy leaving lights on, invariably, someone would turn the switch off. There is only one circumstance that I can think of that would not make this completely transparent and that is when the in line module had the lights off and someone wanted to turn the lights on with the switch. It would require flipping the switch off and then back on to get the local control to kick in. At least I could always make sure the lights were off though.
Also, the WS467s are also a lot cheaper!
I, also, do not understand how function of the XPDF differs from the function of the XPD3 (I know that one is a switch replacement and one is an in line module, but it appears to me that function identically).