Gambling is a form of entertainment & a recreational activity. Like many other recreational activities, there are costs involved. When you gamble, the odds are against you, and on average, people lose money. Similarly, people spend money going to the movies, or to a nightclub, or to a ballpark to watch a game, or bowling, and so on.
If you would condone a person going to a professional football game and spending $100 ($50 for a ticket, $10 for parking, $40 for food and beverages) then why would you suggest that it's a problem for the same person to go to a casino and lose $100 playing blackjack or slot machines? Furthermore, it is assured that the individual will spend the $100 at the game, but there is a possibility that he or she might win at the casino.
Any responsible adult is capable of making decisions of how to spend their time and money. Yes, some people have an unhealthy urge to gamble, and some people are addicted. Some people even gamble away their home, car, and all other possessions. But then, people get addicted to cigarettes and develop heart and lung diseases. Should we ban cigarettes? Other people drive irresponsibly and kill or injure themselve and others as a result of reckless driving. Should we ban cars?
People get an unhealthy fixation on spending money frivolously on things like clothing and jewelry. Should we ban credit cards and shopping malls?
Casinos employ millions (yes millions) of people. The Foxwoods Casino in Connecticut has over 10,000 employees, and that's just one casino.
So those are some positives.