Economics does not attempt to make value judgments about how people spend their money, because this would be impossible. For you, money spent attending a pro football game might be a wasteful and irresponsible extravagance. To me, it might be an important component of my quality of life.
How, then, should we value my purchase of a pro football ticket? Simple–we value it at the price I paid for it, and leave moral judgment to others. A dollar spent on a prostitute or pornography is the same as a dollar spent buying books for children.
With respect to casinos, money changing hands between bettors is an economic wash. Income for the winner offsets a loss for the loser. However, the money which remains in the casino’s hands is recorded as a purchase, as revenue to the casino, and thus as one component of GDP. (Remember that gambling losses can only offset gambling winnings.)
By definition, therefore, and tautologically, casinos are good for the economy. They generate economic activity.
You may say, were it not for the casino, people would spend that money in other ways. Maybe, but you could say that about any business, so it’s a meaningless argument.
You may say, casino activity isn’t as good as other economic activity, because it’s immoral, or because it’s not based on value-added production. That’s your opinion, and you’re welcome to it, but once we start down that road, we’ll be arguing all night. Casinos generate economic activity, period. They’re good for the economy. They may not be good for society–I’ll leave that argument to others–but they’re good for the economy.