1) The WX
radar on the 744 will not detect turbulence in a clear sky. In the WX
/TURB mode it will detect turbulence associated with precipitation. However, it is not all that good, sometimes you have to take the gain out of auto and really play with it to get a decent return.
2) Remember that in filing a flight plan it doesn't really mean that's what you're going to fly. However, in the original posting, the departure is most likely the only one that will work. But, that doesn't mean it will be actually flown. If there is weather, LA
centre will certainly provide either a direct routing or vectors around the weather.
3) As far as the crew taking weather into account in flight planning, it is true to a degree. In today's airlines, dispatch generally does all the flight planning and they take a look at the weather, both enroute and destination. No crew shows up in ops and starts from scratch on an international long haul flight. There just isn't enough of time to do that.
4) Finally, please remember, the weather forecast is just that, a forecast. In some situations the forecast I am using will be 18 hours old when I arrive at my destination. Quite a lot can change during that time.