If you're looking for some heavy philosophizing on the subject, read writings by doctors who work with addicts, the homeless, the terminally ill, or primary care docs in rural or urban underserved areas. If there's something to be said for combating a feeling of futility, they'll say it.
I'm reading In the Realm of Hungry Ghosts right now. It's written by a family practice MD, Gabor Mate, who works with drug addicts in Vancouver. He expressed the feeling you seem to be addressing in your post. It's something he focuses on quite heavily throughout the book. "What have I to offer this young Native woman whose three decades of life bear the compressed torment of generations? An anti-depressant capsule every morning, to be dispensed with her methadone, and half an hour of my time once or twice a month."
Personally, I'm an idealistic and starry-eyed pre-med. I'm reading stuff like this in order to come to grips with the fact that being a physician apparently does not equal curing every patient with whom you cross paths. I guess I've settled on the fact that there is important and worthwhile work to be done, even if you can't "save" everyone. Providing help can mean a lot of different things.
