In my street photography class, I encourage my students by saying that all things are photographable in any way. And this is true. I encourage my students, as well as myself, to go out into the world with camera in hand and no preconceptions that could interfere with openness to taking pictures.
But I have one exception to that anti-rule, and that's street people. I feel that photographing them in their poverty is taking advantage of their difficult situation, and that they are not necessarily there voluntarily. Since for many people sleeping on the street it is their "home," I feel it can be argued that photographing them is an invasion of the little privacy that they have. So, I do not go out of my way to photograph them. In addition to the moral issues surrounding photographing street people, they're too easy to photograph. Where else are they going to go?
I just received email from a fellow street photographer showing his close-ups of a street person with a sign around his neck saying "tired and hungry." They were technically good photos, but add nothing to the millions of photos of street people that have already been taken. The photos didn't put money in the guy's hand so he could buy a sandwich. And I don't think looking at the photo would motivate someone to go out and find a street person and help him or her out. Supporting a local soup kitchen would be more socially responsible.
When I see a street person, I put down the camera and smile at them. If I have some change in my pocket, I'll give it to them and wish them luck. There's something wrong with photographing someone who can't afford their next meal and who lives on the street with a camera that costs more than what they might collect in a year on the street.
I'd rather photograph tourists looking stressed out, road warriors yakking on their cell phones, and office workers grabbing a smoke because it's been banned in their offices--or, really, anything else--than street people looking hungry and begging for money.
Sorry for the rant. Just had to get that off my chest.