In terms of the usual rules of grammar, the second comma is unnecessary, but that second comma is what makes this an elegant, powerful sentence. The second comma introduces weight to the word "lesser" by conveying how the word would be spoken. If an actor, speaking the words from a screenplay, said, "He was feared by other, lesser men," the line would come out flat. The actor reading "He was feared by other, lesser, men," would know to speak the word "lessor" with a lower voice, giving it more weight. Parentheses would achieve something like the same effect, but that second comma does it more powerfully.