I think that one purely economic case that can be made against inequality is immobility of money. Rich save higher percentage of their income than does middle/lower class, so, while this capital is being invested, it is not creating a lot of additional demand. Lack of demand is what US is going through right now, hence we have the stimulus bills and other government spending that is used to prop up the demand. Had this money been spent of buying domestically produced goods, we would see companies starting to hire and unemployment going down. The money needs to be spent to keep the economic system going and with concentration of wealth it is not happening as fast as we would like it to be.