Drop bars give at least 5 hand positions if you care to use them. Flat bars have one unless bar ends, and then 2 or 3 at most.
This helps reduce hand and wrist fatigue.
If you do want flatbars anyway, the hybrid forum is definitely the place for you. Really fast and light flatbar bikes do exist-- Cannondale Quick 1, Trek FX 7.9, Specialized Sirrus Limited, and many others.