Adding liquidity to markets, which speculators do, is not zero-sum.
... Probably I don't have a deep enough understanding of the value of liquidity in asset markets or a deep enough appreciation for their role in creating market prices, but it seems to me the world would get along just fine without them spending all their time and effort competing against each other. ...
Let me explain the value of liquidity then, but first establish that you agree capitalism is a good thing? I would imagine you do. I also imagine you believe legitimate goods and service producing businesses are a good thing, both economically and for the world in general. If this is also true then the stock market, with its intended function of providing capital to promote business opportunity, is also a good thing.
Let's use as example Starbuck's coffee. Starbuck's utilized the stock market to grow a good local idea and establish a worldwide presence. In fact, one of their incentives for employees is owning stock in the company.
Starbuck's stock was hit hard by the economic downturn as the belief was people in recession would scoff at $5 coffee.
Imagine the majority of stock was held by mom and pop investors and speculation didn't exist at all. The likely result would have probably been a total crash of the stock's price as sellers wouldn't find any buyers. Many amateur investors would be wiped out with little chance of recovering their investment.
While investors do accept some risk of loss by investing in stocks, such harsh market conditions would likely scare off most mom and pop types. Such a market would be more rigid, less friendly and appealing, and therefore smaller.
Speculators, on the other hand, look for such pricing opportunities. Speculators would (and did) help establish a floor to the falling share price by offering a 'buy' side. Starbuck's stock has not only recovered but is now at all time highs.
This is a real world example of how speculators' liquidity can allow the stock market to be more appealing, and therefore larger. That's not zero-sum.
Could the world and market get along without speculators? Certainly, but that wasn't the question. Your argument is only having a "positive-effect" on the world, and that speculators do.
note: currently the stock market does not operate in true free market and is distorted by artificial money inflation, which reduces the need for speculators' liquidity.... I wasn't thinking of competition in general as being a zero-sum game, because in general it is not. Competition drives efficiency, and efficiency (creating more by using less) is what makes the world a better place -- assuming that the thing you're making more efficient has a positive effect on the world. Competing to build the most efficient bomb or assault rifle is not a world-improving activity. ...
Would being the (smarter) side with better bombs and assault rifles be considered preferable and world-improving if they are used to stop a Hitler?