The commonly agreed opinion on an income tax system is that it is necessary in order to reduce inequality in wealth. That is a total lie.
What the income tax does is tax the production of goods. This implies that it makes the production of goods more expensive and that fewer goods will be produced overall. This benefits the rich, who have the most accumulated goods, at the expense of the poor, who have the least accumulated goods, by increasing the price of those goods. What the income tax does is protect the value of the large fortunes and prevent the small fortunes from becoming wealthier.
The only politicians who support the poor are those who wish to abolish the income tax and all taxes on production.
The fallacies of intellectual communism, a compilation - On the nature of power
I agree.
The progressive income tax and the welfare state that often accompanies it can be summarised as taking away money from the productive population and giving it to the un-productive. Irrespective of the fact that the government should have no right to do this, as people should be equal before the law, the economic implications on their own can be disastrous. This is because tax brackets have two other negative consequences that are not often mentioned.
I would add that progressive income taxes are a tax on social promotion. If someone is poor and wants to invest on his human capital in order to get a higher income in the future, most of this income will be transferred in the hands of the ruling class, thus reducing the incentives to improve one's own conditions. It is almost impossible to estimate the real long-run costs of different taxation schemes, but this factor seems relevant. I would add that this is one of the peculiar cases where reference to eventual historical regularities may be useful: there may be some empirical work on disincentives and will likely find some data to substantiate and quantify the theoretical reasoning.