I'll tell you a few reasons to not use drop springs
A long light spring rate is the way to go. It stores more energy for launces. It alows the front to rise rapindly and depending on how the rear is set up (relationship to antisquat) it will help plant the tire harder allowing more seperation in the rear.
Another thing to take in to consideration with a stiff spring is the fact that your tire does not stay with the road as well. A light spring allows your tire to follow bumps better and not throw you out of a corner when a bump is introduced. Vehicles actually handle much better with a lighter spring and stiff sway bar set up. Again allowing the tire to follow the road better. Herb Adams who is a road race / circle track guy talks about this in his book Chassis engineering and Dave Morgan NHRA Chassis vetran also talks about under springing a car.
Also, just because it is a lighter weight spring does not mean you can't carry the same load. If your not itroducing coil bind or smashing down on the bumps, there is no advantage to a stiff spring IMO.
Not to mention a smoother ride.