Stock Investing as a Zero-sum Game?

While doing research on machine learning and data mining, I encountered an online discussion on whether or not stock investing, as well as computer-assisted stock picking, is a zero-sum game.
The discussions were initiated because of a New York Times article "How a Computer Knows What Many Managers Don't". There are quite a few good and informative points raised by contributors that are on either side of the debate.

My personal answer to the debate is: it depends. It depends on the type of the investors concerned (speculators, manipulators, serious value investors, etc.) and investing time-frame (day-trader, short- and long-term traders). When a company continues to provide goods or services that the world needs, its value (and the stock price) increases over time and everyone could be a winner, though a better investor would win more. One such example is BHP over last three years. Almost everyone has been making money from BHP not because other people have "lost" money, but rather because the Company's value has increased due to the world's demand for its products.

Even if a trade is zero-sum, as it seems so on a short term basis, the trade increases the liquidity for the market, which is positive for the whole system.

One may compare stock-trading to sports. Each of individual games in a sports competition seems a zero-sum game: one side wins and the other loss, even though the outcome is not zero-sum for the players: the winers take more home than what the losers loss. For the rest of the world, it provides entertainment value and, arguably, improves the health and well-being of human kind. Stock trading is a competition, participated by millions of people worldwide. Even though each trade may seem a zero-sum game, the system rewards hard-working, highly-motivated people and with better skills or tools, machine-learning tools included. In addition to liquidity, a trade, zero-sum or not, provides entertainment for everyone (that's what you are doing anyway when you watch CNBC), and rewards managers and company employees who work in the company and, hopefully, offers incentives for them to develop even better products and services for the rest of us.

The debate will never ends, but in the mean time, let's work hard and try to stay on the positive side of the competition.