I suppose it depends on where you live. In my city (Cape Town) beggars can make up to $400 a week at certain high-traffic spots. People who give them money are enabling them. Also, in my city certain organisations exist to help the homeless and IMO it's better to donate to these groups instead.
In many places in South Africa you are encouraged NOT to give money to beggars.
http://www.capetowncid.co.za/docs/6268/Image%20from%20Best%2...