Originally Posted by
abdon
To be honest you can do a descend on a flat bar. You can't get as low as with drop bars but you can bend your elbows enough to matter. If there are switchbacks or any condition where full speed ahead is not feasible, an upright position actually helps to control your speed without burning your brakes.
Descending in the drops is also about being able to put more weight in the front wheel, to get more control of the bike, besides aerodynamics.