Agreed. If you are using a single X10 motion sensor to control a single or multiple lights and you don't want to use manual light controls (other than for a backup override), then I would recommend using the built-in light detector to determine whether the light should turn on based on motion.
I use a pair of X10 sensors to control a single light and since they are not linked in any way, I have to decouple the sensor from the light. I use another X10 motion sensor for a different light that has no manual local control and just let the motion sensor work as it's designed. That light is far more reliable and quicker than the decoupled light. The only caveat is that sometimes an OFF command is missed so the coupled light could potentially stay on. To negate this issue, I use both a timeout (automatically turn light off if no motion or OFF command seen in xx minutes) and a CRON event (turn off lights at dawn) as a backup.