I believe the author's hypothesis is that certain ages are in need of particular virtues, either as inspiration, as recognition of important societal goals, or sometimes as the expression of undercurrents that have not yet swelled to the crescendo of realisation. His chapter on heroes during various times when wizards and witches were particularly oppressed is said to be quite groundbreaking.

And yes, Ravenclaw Corner. Quite. One must look to the future, after all.