@pior's advice is generally good for organic character art, where most shapes are pretty blobby, soft and don't need many hard edges. However, for precise hard surface stuff, using hard edges along UV seams does have benefits, and this technique is not the result of poor planing nor will it introduce other problems down…
I'm not certain, but i think some of the confusion regarding the "don't harden all UV island edges" comes from the difference between the mindset of smoothing groups vs hard/soft edges. If you are setting uv islands to be separate smoothing groups then that won't harden all the uv borders they are different things. For…