Again I think it has to do with intention and what one wishes to accomplish. If pictures of homeless people helps raise an awareness of their lives and challenges, and if it's done well (like NB23's photos above), it could certainly spark interest among people who are better off to lend a hand and find ways of helping out.

I don't think the act of photographing homeless or ill-fortuned people is disrespectful. But the intent must be scrutinized, and I hope that there is a will somewhere within the conscience of the photographer to do good by these fellow humans.