Just to stop the bun fight guys ....
Quite correct - the weather radar is normally put on during the taxi out to the runway - great for looking out before take off into Wx - planning diversion from the standard departure etc....
In the case of the KLM seeing the other a/c on weather radar - not a chance - all he would see was ground clutter - a return of mainly red on the whole screen with the dish pointing along the ground.
In our company we have the radar tilted to 5 degrees up to avoid ground clutter.
Hope this helps the thread
Cheers WM