Paul Graham published a simplified version of his essay on income inequality. Here's my simplified response:
It is true that many socially worthwhile activities (like developing technology, and starting startups) produce income and wealth disparities. And a certain amount of income and wealth disparity is not a bad thing. In fact, having no income and wealth disparity is a bad thing. We tried it. It didn't work. No reasonable person disputes this.
However, too much income and wealth disparity is also not a good thing. I hope I don't have to convince you that if all of the wealth in a society is controlled by a single family or individual, that's not good. The optimal point lies somewhere in between those two extremes.
Today, the top 1% control fully half of the planet's wealth. That is awfully close to the second extreme. It's very unlikely that this is where the happy medium lies.
It's particularly worrisome because, as income and wealth disparity increases, there is a positive-feedback effect that tends to drive it further and further towards the second extreme: the extremely wealthy use their disposable income to buy political influence, which they then use to get laws passed that favor them at the expense of others (e.g. through regressive taxation). This lets the already-wealthy accumulate more wealth not by creating more value, but by collecting economic rents. In the long run this leads to oligarchy and possibly even economic and societal collapse. We are nowhere near there, but by the time it becomes apparent that this is happening it will almost certainly be too late to do anything about it. The time to act is now, before it becomes an even more serious problem.
(Paul Graham, like nearly everyone in today's politics, fails to appreciate the near-universal validity, at least when it comes to social and economic policy, of Ron's First Law: All extreme positions are wrong. :-)