Actually, a magneto draws a HUGE amount of power. A magneto, like an MSD or Mallory unit, is not reliant upon a battery. Alky and Nitro motors run magnetos for a couple of reasons. First and foremost is the amount of current they deliver. We run a 44amp MSD mag on our race car. Top fuel cars run 2 of those 44amp mags. The primary reason for running a mag is that when running a motor with close to 50lbs of boost, the electrical resistance during compression is so high that the plugs won't fire without major power. In a nitro motor, we run our plug gap at 0.017" and a 44amp MSD mag... this is the only way to over come the resistance. You cannot turn a 44mag by hand. It takes a tremendous amount of power just to turn one of those things. However, the outcome is just like a blower, it takes a lot of power to turn one, but the boost they provide far outweights their parasitic draw.
The battery on our race car is there for two reasons. It's there for the starter and it's there for the computer. Other than than, the race car is capable of running without an outside power source such as a battery. There's only two ways of turning off a pro car, such as ours. 1) you kill it's fuel supply... and 2) you ground out the points box on the mag. Simply turning off the battery switch will do absolutely nothing.

Bracket racers use alternators because they are cheap.
An MSD Pro Mag 44 like we run is about $3500 with the points box and mag. Also, plenty of bracket racers don't use alternators at all. They simply plug in a battery charger when they get back to the pits and let it charge while they wait for their class to be called again. I could see this as a problem in a constant round robin race, but normally they are fine.