For reference - I lived in a very clean SRO in a co-op building. We all kept it looking good.
Absent additional research, my prior is that homeless disease vectors are probably worse, since people in that position have fewer shelter options from the elements and more direct exposure to the public.
Houses don't have to be perfect, they just have to be better than what we have right now. I mean, even in a bad situation... there are rats on the street. It's very difficult for me to imagine someone in that condition looking at a house and saying, "no, the street is cleaner."
Maybe there's an extra perspective I'm missing.
Is this abused? Sure. But its a lot better than living in a tent on the street where you are guaranteed rodent issues along with far worse health hazards.