Simply leaning over does not put you in proper cycling position (where the arms are just slightly bent, wrists are straight, and back is at a 45 degree angle). Drops allow for one to ride in proper position for an extended period of time, so that discomfort is eliminated and performance is enhanced. While leaning over on bullhorns or a straight bar may work to get you through a brief headwind or power up a hill, it's definitely not the same as riding the drops for 10 miles. If you did ride bent over for ten miles on any other type of handle bar you'd probably begin feeling it (or not feeling) in your hands and lower back. You shoulders may also become tight.

A good set of drops beats any other type of handle bar for road racing IMHO.