the 'flexibility' and cutting ability of japanese blades is also a myth. if you are doing cutting with a katana, especially on stuff like bamboo, if you make a bad cut, you can bend the blade. even a master can have a bad day. with a soft spine and a hard edge, it is less likely to snap, which can get you killed in battle, a quick bend the other way can restore a bent blade, at least well enough to preserve your chances of living long enough to replace it.
japanese used layered and laminated steel to make the best of the best of the steels available to them, producing art accidentally in the process. modern mono-steels produce better swords for cutting competitions, less likely to bend or break and if made correctly, sharper - the mono steel allows a wider blade and thus a more acute edge angle.
they generally do not have bohi (fullers) to reduce cutting friction and move more weight balance out to the tip to improve the inertial forces. quick recovery to defend or re-attack is not a major requirement when cutting bamboo or pool noodles, or tatami mats.
fullers improve the strength to weight ratio, and are used for the same reason we use I-beams. a box beam (hollow square x-section) maximizes the lateral and vertical stiffness and strength to weight ratio, but it's be rather difficult to make a hollow square sword - 90 degree edges do not cut well (tho they may work for a stiletto or a pure thrusting blade/ small sword or cane sword).
fullers are used in sabres and swords because they work if done correctly, improving the balance and handling. bohi on samurai katana are also used because of the neat noise they add if you cut correctly (and don't bend your blade).
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