I just saw I forgot to add that it does of course depend on the country and the tax laws. In some countries there is no tax whatsoever on gambling while in others there might be a pretty high tax. What is even more important is from what angle are you looking at the problem? Nationally? Globally? Here is why it is important:
First of all I would say that intuitively an activity where your loss is my gain and your gain is my pain doesn't sound like it could ever be a net positive in terms of social cost.
But people are aware about the risks without being forced to play. And they wouldn't risk losing money if there wouldn't be chances of growing this money at same time. Actually we are hostage of the *your loss is my gain and your gain is my pain* effect in our daily life. Isn't that what governments do charging heavy taxes over basic needs of the people? With the difference in gambling you can choose participating the activity or not.
But apart from that, if we look at a country with a gambling industry (take Nevada, Vegas), of course the statistics will show that Vegas is in an awesome position in terms of social cost. That is the national perspective. But what about the countries that the players came from and spend their money in Vegas? If they win money in Vegas they bring it home, spend it at home and reduce social cost. But no matter how you look at it, it is obvious that that had to come from somewhere in the first place.
So moneys balance can't be the basic measure for social costs here, I think we agree on that.
Foreigner gamblers going to USA to gamble considerable amounts of money are rich businessmen, investors, speculators on their countries. They won't cause externalities to their local governments. There are some common tourists too, but the amounts they bet are insignificant. Of course they could be gambling on their native places, but the local governments should legalize it first, right? The irony here is that they don't want to legalize gambling because it would be harmful to their citizens, but at same they complain people are going abroad to gamble.
One of the questions that remains is does the winner spread so much positive energy that his surrounding society becomes more productive, and does that net out or even exceed the serious cost (maybe he lost it all) incurred by the loser? While the marginal effect of joy is strongly decreasing (that is scientifically proven), the loser could literally lose the ground under his feet, become an alcoholic, lose the job and destroy the family, cause the kids to get off track, maybe is forced to attend psychotherapists and what not.
I don't know, just some thoughts on the issue. I wouldn't look at isolated geographical areas. It is like: hey I got some statistics from Monaco, let's see if gambling their supports or undermines the socioeconomic system...
I think the effects of gambling are positive, yes. Millionaires hate to pay taxes and avoid countries where the rates are high, taking away with them all their wealth, so gambling is the way for a country to get money from them through free will. On long run they lose, but also have fun and won't miss the lost money as they have much more to spend (what also include the maharajas of the judiciary and other public employees).
Furthermore we can't stop doing things we like just because some people don't know limits. This way everyone is being punished for few people's mistakes.