Often times homeless people are just guys in a coat. While the image does not illustrate homelessness, it's a decent enough portrait. That's kind of the challenge of photographing the homeless, how do you illustrate the person and their social status without it being solely about one or the other. I would argue that it is far less ethical to try to use a homeless person to represent all homeless people; when photographing the homeless, we should be capturing the individual's essence, which to a large degree may include homelessness, rather than trying to capture the essence of homelessness through an individual.
Another issue I have with much homeless photography is that while the ordinary people can go home and live a life of privacy, the same does not apply to the homeless. The homeless truly have no expectation of privacy, and it is wrong to exploit this characteristic of their living condition for the benefit of the photographer. But this isn't a cheap shot of a homeless man sleeping under a bridge. So I don't think that it is exploitive in that area either.
Unless you've been homeless, and have first-hand experience knowing what it's like out there, i'd say you really have no right to say what is acceptable.