Isn’t it more useful to be able to cut your enemy from either side of the sword? What were the Japanese’ rationale for having a unilateral sharpened side only?
Just some thoughts to ponder on after playing the Witcher 3 (medieval Poland) & Ghost of Tsushima (feudal Japan) back to back. Both games rely on sword-based combat, got my brain wound up on how Eastern vs. Western sword combat differed.
I can't address the design process of Japanese swords, but I can provide some thoughts on the second question:
How did this difference significantly affect medieval combat?
Not at all. None whatsoever. Swords, in general, don't matter that much to battles.
The key weapons on a medieval battlefield are polearms and missile weapons. The exact weapons change over time - at Hastings in 1066 the spear would be the weapon of choice, mostly used with a shield, while at Courtrai in 1302 the Flemish bristled with pikes and goedendags (a type of heavy club/spear) and at Nancy in 1477 the Swiss made good use of pikes and halberds. Mounted troops, of course, favoured the lance for their polearm. Individual soldiers might switch to shorter weapons (swords or even daggers) if the battle devolved into a tight press, but the exact weapon that came to hand would rarely a relevant factor in this sort of 'bad war'. Instead cohesion, morale and weight of numbers would typically be the deciding factors for which side triumphed.
At this point, you might say something like "well that seems fair enough, but what about an individual fight?" Again, it just doesn't really matter. Single-edged swords weren't as common as double-edged ones in medieval Europe, but they certainly existed. Neither version was able to completely displace the other - sabres are still switching between both patterns when they were finally abandoned in the early 19th century.
We can get a direct view by checking fencing books (fechtbuchs) through the ages. Writing in the mid 1400s, Hans Talhoffer interchangeably shows double-edged swords and single-edged messers in MS Thott.290.2º, switching from sword and buckler to messer and buckler within a page or two. In the early 16th century, Andre Paurñfeyndt writes:
THE SECOND CHAPTER teaches how one should use the messer advantageously, which has exceedingly increased usefulness because of its versatility and which is a predecessor and main source of the other weapons that are used with one hand, such as the tessack or the dagger, the straight cutting sword or the thrusting sword and many more one-handed weapons which I leave out for brevity's sake.
(Trans. by Robert Kraaijeveld, from Wiktenauer.)
Notice how he addresses the messer (a single edged sword) but explains immediately that the techniques you learn here will be directly applicable to any one-handed sword, including double-edged ones. This theme continues past the medieval period - the double edged 'broadsword' and single edged 'backsword' are used in the same way according to contemporary authors.
