TL;DR: Defending HFT’s argues for rewarding market participants who simply jump in front of the line b/c they see other people doing it, at the expense of participants who have worked hard to decided which line to get into.

Here's a more fundamental question to ask:

What kind of behavior do you want to reward in the stock market?

If you view the stock market as a bunch of numbers that wiggle around, where "information" is conveyed by the actions of other people participating in the market (ie, what they’re buying and selling), then the HFT's play a legitimate role in spreading that information around.

If you view the market as a place to buy and sell businesses (equivalently - shares of businesses), where capital is allocated to businesses commensurate with the value they provide to society, HFT's are parasites on the effort of others.

There are participants in the market that spent an enormous amount of time, diligence and thought into deciding allocating capital to businesses according to the value they ascribe to the businesses. For these participants, the signal they send by putting out their orders is currently taken advantage of by other participants who spend enormous time, effort and money into trying to “tap into” the result of the efforts put forward by the first group and rush to another exchange to buy up that company before anyone else gets there.

So: Do we really want to reward all of the effort spent building faster machines, communication links, and algorithms to jump right in front of someone in line? Or do we want to reward those people who are actually determining which businesses have intrinsic value and which do not?

It's easy to hate Einhorn or Ackman b/c they're rich. What's important to keep in mind is that they are custodians of and act on behalf of thousands, probably even millions of other people to improve their financial situation. Pensions, endowments, sovereign wealth funds, etc make up the bulk of the funds that Einhorn, Ackman and others invest. The fact that they're rich an indication of their legitimate hard work at researching and allocating capital and enhancing the financial well-being of their clients. (Note: yes, there are always bad apples that get rich by illicit (read: insider) means, but that is an exceedingly small amount of participants and over time they usually get caught).

Warren Buffett has often said that a perfectly good stock market would be one which is open only one day a year, where participants put up orders based on the value they actually ascribe to a business. If those orders cross, then trades occur - all in one day.

Makes you wonder what would happen to society in that market. Would people would be able to find a way to channel all the frenetic activity currently employed in chasing prices and watching others’ actions into a more productive endeavor?