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PoW is as useful as it is necessary. When PoW seems to be useless the question one should ask is, "Is it [the PoW schema] necessary?" The necessity of PoW, I have found, is directly proportionate to that of its coin. In light of this and the discussion here, it would follow that Bitcoin might not be necessary and that, because of that, its PoW is widely deemed unnecessary and, thus, "useless."