Here are a few advantages of a 3.33:1 vs a 4:1. Warning: Not a short read.

Advantage 1:
A 4:1 ratio means the same tooth on the pinion is hitting the same 4 teeth on the spur and only those teeth. If you have a blemished tooth (either from the factory or debris somehow messes up the face of a tooth) or don't have any lubrication on a patch of the pinion gear, the gears will wear out more quickly since the same deformed tooth only has 4 teeth to interact with or the lubrication won't be spread across the entire ring gear. This means that these sets of teeth wear into each other differently. If you remove the diff and put it back in on a different set of teeth, they will need to re-break in again since they aren't hitting the same teeth they were before. Lets see how many times I can say teeth in this paragraph. This can lead to noisy diffs and premature failure of the teeth.

The 12 tooth pinion and 40 tooth ring are better, but still not ideal. Each tooth on the pinion will contact every other tooth on the spur gear (both divisible by 2). An ideal spur and pinion combination creates the situation where every tooth on the pinion eventually hits every tooth on the spur or ring gear. An 11 or 13 tooth pinion would do this with a 40 tooth spur and is exactly what many other manufacturers use in their cars. 3.08:1 (13:40) is a common ratio along with 3.38 (44 tooth spur, 13 tooth pinion). Every tooth hits every tooth and they all break in identically.

Advantage 2:
"It accelerates better" and "spins the center drive line slower". Those two statements are both true and directly connected. Think about it like this: Lets say at full throttle, your car goes 40mph. At that speed, the motor is maxed out on its rpm and the tires, front, and rear diffs are all spinning at the same rpm. At a 4:1 ratio, the center driveline is spinning 4 times faster than the front and rear diffs. Now lets jump to the 3.33:1 diff gears and go down a couple teeth on your motor to keep the same top speed. The motor, tires, and front and rear diffs are spinning the same rpm as before. However, the center driveline is spinning 20% slower. That means less energy is required by the center driveline to accelerate it to max rpm. Since kinetic energy is a square function, it isn't just 20% less energy to spin up the center driveline. It is 36% less energy. That directly translates into more acceleration and longer run times.

The drawback is that the center driveline is under more torque than before. This may cause additional wear or it may not, since rpm plays a roll in how fast parts wear out as well. This would require a more tedious analysis to determine. Given the straight driveline, I'm more inclined to think there will be additional wear on the pins and cups. However, in the nitro buggy with it's offset center diff, there might be less wear since there are fewer revolutions for the pins to slide against the cups with.