Just as pistons fail, so do clutch baskets. They groove up due to the impact of the plates on the basket fingers. This is largely the result of riders using the conventional multiplate clutch as a torque converter, something it was never designed for. Twist and go scooters have such devices in place of gearboxes. The multiplate clutch was designed to facilitate setting off and gear changing. Using it to bridge gaps in gear ratios and help when the engine is getting bogged down is beyond its design brief, and whilst it will manage this for a short while, the end result will be deterioration and ultimately failure.
Motocross designers have always understood this and attempted to reduce the effect of plate impact on the basket fingers. Early Yamaha YZ125s had 16 or even 20 peg baskets, but the die cast aluminium was brittle and as power outputs increased, the pegs had to be widened and their number reduced . Clutch baskets were on their way to becoming a disposable item. They weren't going to break, but they wouldn't last very long before grooving.
So how does a billet basket such as Aoki help. Well obviously if it is made from a harder material, this would be a "good thing". Well Aoki baskets are made from 7075T6 alloy, the strongest and hardest commercially available material. Specifications of the material can be found on this link :-
The problem with all high strength aluminium alloys is that they deteriorate significantly at temperatures above 170 -200 degrees. This is why one rider will experience finger grooving after just one race, whilst another will get a full season out of a basket. As you will see from the specifications laid down for 7075 alloy, it is produced almost exclusively for aerospace applications and quality control for this particular material is so stringent that it is virtually impossible that a 10 ft bar could have, so to speak, a soft inch in the middle. If a basket grooves up, it is not a material or manufacturing fault.
If a rider is experiencing grooving of the basket it is definitely an overheating issue. Obviously low oil level will cause problems, but generally a rider will need to modify his riding style or accept the need for frequent replacement.
Coatings such a hard anodising (This results in a sludge brown colour on 7075 alloy and a more attractive silver colour on 2104 - if your basket is red, blue, gold etc this is decorative not hard anodising !) do help, as does chrome plating, but ultimately it is the core strength of the alloy which matters.
Well we could say that it saves weight, which is true, but the honest answer is that it makes them easier to to produce and so reduces the price. There is simply no need for full depth splines. If a kickstart gear is going to spin, it must shear the splines at the root. This will happen regardless of how deep the spine is. Kawasaki have for many years run baskets with no splines, just relying on the kickstart gear to cut it's own very shallow spline on assembly.