Everything in moderation is key. A gambler is not harmful just for doing it once, but when it becomes an addiction that is when it puts them at risk. They become obsessed with the gamble and risk all their money causing them to lose their homes, food, and potential well being.
Gambling can be a casual hobby, or an addiction. There are many people who have gambling problems, but there are just as many (or more) who enjoy doing so as a hobby. It really depends on the individual. If you have the extra money to potentially lose, and it doesn't interefere with your everyday life, then what's the harm in it?