One man can only eat so much bread. As wealth becomes concentrated, consumption of goods and services decreases (because fewer people have money to consume and the few people who do never consume enough to make up for the lack of consumption from the majority sectors).
As consumption decreases, production decreases (because nobody pays to make stuff that doesn't sell). When production decreases, people lose their jobs or become underemployed (because labor is just another input and gets reduced when production gets reduced).
Rampant unemployment leads to smaller incomes and, cyclically, a decrease in consumption. And this leads to the problems just mentioned. As the process continues, consumption is cut not just for accessory goods/services (which are also important to an economy) but for essential goods/services as well: food, education, healthcare, housing, etc. This leads to a lower standard of living and when it occurs on a large scale (as is increasingly the case in countries such as the U.S.) threatens to plummet a society's Human Development to dismal levels.
To decide whether economic inequality is bad, folks merely need to ask themselves whether they want to live in a first world country or a third world country.
When economists measure economic growth and resilience in our economy, they mostly measure consumption. If less people are able to buy something, that means there will be overall growth. While income inequality can be a complex subject in terms of measuring how it happens, the fact that is here makes it important to fix as there will be economic growth. A billionaire mostly saves and invests money that can lead to the next Apple which is obviously good. But say that he lived in a country of a million people that together earned a billion dollars as he himself or herself does. Would those million people be able to buy the product that was invented? Even if it sold, how much would it sell? Would you not say that if the income was distributed in that country much more efficiently and equally, would there not be more people buying that product along with more economic growth as more people can have families which leads to future economic growth along with the ability of people having a market opportunity to set up a store that would sell those products which would then hire people. Does that sound like a bad idea to you? Now, we can argue about how it should be fixed but it has to be fixed. Answering your question, it is bad for economy. Some inequality is good but too much like today as it was before the Great Depression says a lot about what could be ahead if we do nothing.
Economice inequality is unimportant. Even more it is desirable and it is good for economy. It gives incentives for people to be innovative and create a better world. Rich people are rich because they are smart and hard working. Poor people are lazy and dumb, so we can't make them smarter.
I think that the way the U.S is now is fine. Why would we even want economic equality? Then that would mean that no matter what career we have, or how hard you studied wouldn't matter because were all going to get paid the same in the end. That's actual not equal at all!
Reason being that people live lots of money tend to have responsibilities more than those who don't have as much. They don't use that money for leisure things. Most people who get the average pay don't have as much responsibilities. So there is no such thing as inequality. Inequality is more common in feudalistic, socialistic societies, etc.
Inequality would be those who have gold, and those who don't.