It's interesting to read about that, as I just received a book to review on my blog called, "Good without God." It's a book about humanism. I think you raise some good points here in response to the related subway ads. The most important part of the book so far in my reading of it is that Christians often overstate the consequences of not following God. There are some quotes from fellahs such as Rick Warren stating that there is no basis for morality without God, which we can argue for on some grounds, but at the same time humanists feel besmirched. 

When I read the writings of a humanist such as Kurt Vonnegut, I encounter a lot of reasons for us to be good and decent to one another. Christians don't have the only path to morality. What we do have to offer is a relationship with the living God and a new life under his Kingdom, and I think we lose the more potent elements of our message when we overreach and leave ourselves open to critique from those who believe they can be good without God. Can they be good without God? Yes. 

The issue resides in wholeness and the complete nature of our redemption. We can be good in part without God, but our completeness and our ability to continually act out of self-sacrificial love comes from God.