The answers to your questions are here:
http://www.precisiontandems.com/arai.htm
In short, a drum brake is used on exceptionally steep mountainous descents with lots of switchbacks by most all teams and on steep or long descents by heavy teams, tourists travelling with full panniers, or by folks who just prefer to descend at something less than the higher speeds that tandems can easily achieve where using only rim brakes would cause the rims to overheat which, in turn, softens the tires and causes the air in the inner tubes to expand such that a tire could come unseated and "blow-off" the rim.