While most historical Japanese swords were made with laminate techniques, it is perhaps flawed logic to assume that this was the reason they were high quality. In fact, many rather shoddy blades I have seen were made with these laminate techniques. There is also little reason why it would make a higher quality sword, only one that is more prone to bend and very hard to break (more like taffy) with less hardenable or unhardenable shingane core.
You can achieve a hamon using a variety of principles, but yakiire is the way you do it with traditional blades. Yakiire is also not the only means to produce a hard edge and a soft spine, nor is it necessarily the "best" way (though I believe it has the prettiest effects). So long as the edge cools faster (to form martensite) and the spine's cooling is delayed (to form pearlite) you will yield the coveted hard edge and soft spine out of the quench. You can also temper the spine back further while leaving the edge martensitic, and have useful results.
As far as showa period (wartime) swords, things were fairly hit and miss. The amount of attention paid to the swords varied dramatically, and there were some that are extremely nice, and some that are shoddy in comparison to even Hanwei pieces (no disrespect intended!). They are not Nihonto, and they are not art pieces in the same way Nihonto are. Because they WERE made in Japan though, there seems to be a natural reaction to judge them by the aesthetic standards of traditional blades, which these were often nowhere near.
Some make extremely good martial arts blades, some are not very high quality and would not be very useful, some would even be downright dangerous to use. It really depends on the individual blade, so it is wise to not automatically assume all production blades made during the war are crap.
I sometimes wish I could come across a really sweet showa blade to shape up, if that means anything
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If ya come across a good blade, enjoy it without caring if collectors don't value them like Nihonto!
