Progressive taxes are beneficial because the tax rate stays in conjunction with the changes in the cost of living. The cost of goods and services increases and there got to be an economic tax system in place to stay with the increased cost of transporting these goods to the consumer.
If the rich are rich it should not make that much of a difference if they should have to pay a little bit extra on their taxes especially if it is to benefit their country as a whole. It benefits economically because it allows breathing room for the poor to get out of debt.
Ultimately to get economic growth, we need savings which allows investment. If you're taxing away money from the people who save the most and create the most wealth/jobs in general, we will get less economic growth. It's also not really fair to take more from someone who earns more, that's not 'equality', and it takes away incentives to work/earn a lot (albeit not much in practice with the way top taxes at the moment aren't super high, but if you whacked them up to the 60, 70, 80% range things would likely change).