only half that number is in cash, the rest in USAir stock. If the merger works, USAir stock will go up in the long run. If this merger doesn't work, USAir stock could end up at zero. So it depends on how well you think this merger will work.
This merger would also saddle the "new Delta" with $8B in additional debt, which is not such a good start for a new airline. OTOH, a stand alone DAL would have less than half the debt load and most likely a greater chance for increased share holder value.