Forget the morality of wealth distribution. How long do you think 90% of a democracy can go without an increase to their standard of living before they realize they need to drastically change the system?
http://www.cbpp.org/cms/index.cfm?fa=view&id=3309
If you are looking for a systematic problem with income concentration, look at Figure 2 in the link above. Notice any correlation between the peaks of income inequality?