one reason for folding monosteel is it distributes any imperfections, inclusions more evenly. yes, if any of the layers do not fuse, the piece is junk. one reason swords were expensive. this occurred with bloomery or 'bog' steels which were essentially a spongy matrix of discrete particles of varying composition.
modern steels, it not neccessary as they essentially do not have any imperfections if they were made correctly.
experiments with using two steels of differing carbon content were done, laminating them a number of times and testing the carbon content of each layer. after about 8 folds (64 layers)* the higher content steel's carbon content was diffusing into the lower and eventually (i forget how many layers) it was essentially a mono steel of carbon content intermediate of the starting values.
japanese swords were made from steels from two different sources, one too high in carbon, the other too low. the laminating into hundreds of layers was to mix the two steels, done out of necessity, not to produce a pattern. the hamon pattern along the edge is a function of the heat treatment changing the crystallisation, not the layers. they did use a higher carbon edge section, a milder spine, and a softer core all welded into one, but that is not pattern welding/laminating. the hamon is brought out visible by polishing, not by etching. the areas outside the hamon are polished, not etched, as well. visual elements there are defects, some mild enough to ignore. some mean scrapping the blade and starting over. again, a reason the bl;ade of a master smith is so expensive.
it's like those sticks of coloured epoxy putty, cut off a bit and you have an inner and outer layer of different colour, one is the resin, the other the hardener. you mix them by flattening with your fingers, folding, flattening, folding, rolling, twisting, balling, flattening, etc until the layers diffuse together. not enough folding and you see the layers, too much and they diffuse together and more folding just a waste of effort.
* - related: try folding a piece of paper into layers, how many times can you fold it?
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