This is one of the most intense, bitter and personalized exchanges that I've ever seen in the scientific or technical literature. And it doesn't fit either of the usual explanations for such debates.
Since the subject matter is mathematics, it seems to contradict Benford's Law of Controversy: "Passion is inversely proportional to the amount of real information available". And since neither Simon nor Mandelbrot represented a grouping or sect, whether mathematical or social, there's no evidence here for Freud's "Narzißmus der kleinen Differenzen" (Chapter 5 of Das Unbehagen in der Kultur, translated as Civilization and its Discontents, 1930)
… it is precisely communities with adjoining territories, and related to each other in other ways as well, who are engaged in constant feuds and in ridiculing each other — like the Spaniards and Portuguese, for instance, the North Germans and South Germans, the English and Scotch, and so on. I gave this phenomenon the name of 'the narcissism of minor differences', a name which does not do much to explain it. We can now see that it is a convenient and relatively harmless satisfaction of the inclination to aggression, by means of which cohesion between the members of the community is made easier.
Mark Liberman at Language Log on Herbert Simon & Benoit Mandelbrot, re HS's paper "On a class of skew distribution functions"