Kindermann and Krueger say that a top marginal tax rate in the range of 90 percent would decrease both income and wealth inequality...

The problem with the naive assumption that more money for the government would increase everyones well-being is that governments have proven numerous times to be ineffective and inefficient when it comes to budgets.
  Academics always like high tax rates, because their advantages (good locations, subsidized housing, free tuition for their children) are typically immune from tax. It's a little bit like the French nobles, under Louis XVI, who wanted to raise excise taxes. They, of course, were exempt from them. Currently, the top rate of 39.6 percent is paid on income above $406,750 for individuals and $457,600 for couples. Fewer than 1% of Americans, or about 1.3 million people, reach that top bracket.
http://papers.nber.org/tmp/99111-w20601.pdf