Some people like to argue that more economic inequality is a good thing, because it is a "natural" byproduct of capitalism in a world of "makers and takers," "winners and losers," "wolves and sheep," [insert your favorite Manichaean metaphor here]. However, too much inequality is deleterious for both economics and politics -- for reasons that are intertwined.
Those who amass exorbitant wealth often increasingly use a portion of their gains to capture politics. While the mythological promise of trickle-down economics is that we must not have progressive taxation, because giving more money to the already wealthy is the only way to spur economic investment and innovation and create jobs -- in actual fact the majority of tax cut windfalls go to stock buybacks, offshore tax havens, regulatory capture, political lobbying, and campaign donations. All this is a runaway amplifying feedback loop that tilts the playing field further away from equal opportunity, social mobility, and democratic process.
Wealthy elites seek to preserve the power structures that have benefitted them, and keep them (and their descendants) in the ruling class. It is a slow recreation of the aristocratic societies of old Europe that we fought a bloody war of independence to separate ourselves from. Yet the erosion of civil values, public engagement, and collective will -- largely as fomented by the conservative elite over the past 50 years in America -- and the ascendancy of the myth of "rugged individualism" have conspired to create a perilous condition in which corruption operates so openly in today's White House and Wall Street that democracy itself is in great danger.
Moreover, we have learned these lessons once, not quite a century ago, yet have forgotten them:
"Where there is a crisis, the ruling classes take refuge in fascism as a safeguard against the revolution of the proletariat... The bourgeoisie rules through demagoguery, which in practice means that prominent positions are filled by irresponsible people who commit follies in moments of decision."
-- Czeslaw Milosz, The Captive Mind