To digress from the current photographs (sorry, OP). I have two basic problems with this sort of photography.
#1:
The first is that there's no easy way around the problem of exploitation. You're using people who already have it pretty tough, and you're using the fact that they are having it pretty tough as a subject of photography. Sure, you can pay them, you can be buddies, you can buy them a sandwich, or you can do none of those things. The bottom line is that you are turning what is bad in their lives into a photograph for your own satisfaction.
#2:
The second problem is that we're awash in these photographs. Homeless people are just so interesting to look it, you basically get a moderately interesting photograph for free. Point it at the guy, get it in focus, click. Poof, you have an interesting-to-look-at image. You have, frequently, contributed basically nothing to the image. You might kid yourself that you're Recording Important Social Commentary, but realistically you're not Walker Evans, and anyways this commentary (while genuinely Important) is being recorded far too much. It no longer registers as Important Social Commentary, really, because we see so much of it. This is part of a larger problem of being awash in lazy images. There's lots of stuff that's easy: flowers, macro photographs of.. stuff, the moon, hired models, and we see lots and lots and lots of all of it, and I pretty much hate all of it at this point.