I thought the XPS3 and WS13A were designed for use in applications which would create noise, such as powering flourescent fixtures and motors. That is why it requires being directly connected to a neutral line, so it doesn't recieve the noise from going through the fixture.
This comes straight from the X10 website:
Now You Can Have Both! With X10's decorator wall switch you can now enjoy the flexibility of X10 remote control along with a wall switch to control fluorescent lights, low-voltage lights and higher load appliances like air conditioners, ceiling fans or high-wattage lighting.
The WS13A decorator wall switch is like a heavy-duty appliance module you put in the wall. It works on 120V and can handle up to 20-amps at a maximum of 2,400 watts. This makes it the ideal switch to help you cut energy costs by putting devices like your air-conditioner or landscape lights on a timer while retaining manual control with the wall switch.
* Control ceiling fans, fluorescent lights, pool equipment, air conditioners
* Save Energy by controlling 120V devices at up to 20-amps and 2,400 watts
* Red LED indicates if switch is on
* Works in 3-Way or 4-Way setups when used with Companion Switch (WS14A)
* Responds to ALL LIGHTS ON and ALL UNITS OFF
* Does not respond to bright and dim commands
* Match your decor. Includes both white and ivory paddle switch covers and trim plates