Suppose that 90% of the income went to ?% of the population. This could well be true of some countries, such as Equatorial Guinea.
Pretty much everyone will agree that this would be a pretty piss-poor place to live.
The highest quality of life of a society tends to be in a country where income, education, and opportunities are as equally distributed as possible. Norway and Denmark come to mind.
If this were a bad thing, there would be thousands of Norwegian and Danish emigr?s about, badmouthing their country's government in the fashion that Cubans, Puerto Ricans, Iranians and Venezuelans do.
But the truth is they don't: there are very few Danes and Norskis that immigrate, and those that do mostly do so as young people in their 20's for the sake of adventure. They tend to return within a few years.
An unequal society is a defective society. For purely pragmatic reasons, a large middle class with a similar income is best from nearly any perspective.
Prime Minister Olaf Palme of Sweden used to have the collar turned on his shirt collars and cuffs as a way of showing the people that he was not using his position to be ostentatious or to lord his position over other Swedes.
Conspicuous consumption is bad for a society, regard;less of whether said consumption is by the old rich or the nouveaux riches, whether is is Paris Hilton or Fity Cent.