Fundamentally, one can see people living on the street as belonging to a complex subgroup in our society: one which can be both symbiotic and parasitical.
It is symbiotic in the sense that it provides a culture with which many can identify, especially those who have experienced hardship or difficult home lives as children. In this way street communities can provide a sense of catharsis -- almost a group therapy session. Belonging to a community can provide mental comfort which can seem even more important than physical comfort. In other words, street communities can take our tired, our poor, our huddled masses yearning to breathe free.
However, in many cases they can also be parasitical. Consuming societies' group benefits, in a way similar to herd immunity, while also spreading danger in the community, either through violence or excessive drug use.
I think what many people, both in the street communities and not, may fail to realize is that it is entirely possible that a street community can represent a local optimum, but not a global one. There may be a way to provide people both the sense of community they desire and a way to integrate them into the larger community so that they can contribute in more impactful ways.