besides Jhhnn's typically good points, I'd answer largely by pointing out my basic economic view: that there's a 'Goldilocks' happy medium on wealth distribution, both because there's a human interest/fairness to wealth being reasonably distributed, but also because it increases productivity for society.
Papa Bear's too much distribution leaves too little reward/incentive for productivity. Mama Bear's too little distribution leaves the classic societal model of feudalism, where a few wealthy terrorize everyone else as effective slaves, and there's also too little reward/incentive for people, because the wealth is so tied up in a few hands (look at so many countries around the world filled with peasants and a small wealthy class).
In the Goldilocks model, there are carrots for people to do more, and wealth is distributed enough for there to be lots of opportunity. When you want a CEO to do a good job, it's not so important, however much they'll disagree, whether they're making $2 million instead of $1 million, or $200 million instead of $100 million - both incent them to do more. See other nations' corporations with much less compensated CEO's for comparison.
Similarly, the Walton kids aren't exactly making Wal-Mart a whole lot more productive corporation for stockholders or society; they simply have a lot of stock so they're the wealthiest family in the US, and others run the company while billions are handed to the Walton family for their highly skilled owning of the stock. Multiply them by 1000 and you have the 'ownership class' in the US, in great part.
If you don't tax the rich, the harms are many and large. The rich will tend to get richer, owning more and more of all the wealth in society, which is a direct threat to democratic government. Opportunity, reward, will be reduced.
It's a right-wing myth that every dollar put in the hands of a wealthy person is used for the benefit of society; actually a fraction of it is, while a lot of it simply goes towards owning more leading to more wealth. It'd be more accurate in most situations to say that the same dollar given to the *bottom* does more good for society, where it's spent and helps the economy move.
In short, not taxing the wealthy enough, IMO, increases corruption and tyranny in the nation, and harms the middle class and the standard of living for everyone else. It's simply unacceptable in a healthy society.