[tomiles]

because you can't fit in the extra guy in all cockpits :-) Would be unpractical and expensive to put a 3rd person there just for weather. Good updated meteo from the specialists on the ground, and basic training in and availability of satellite images in the plane.

Like it's said you might loose contact with any ground station when flying in the middle of nowhere or you might lose imaging data of the storm. So both should be there to provide a better standard than the current limited info of printed weather maps in preflight briefings, especially for long flights.