I've actually eaten dinner with a bum once. A guy came into a laundry-mat I was in when I was going to school and tried selling everyone some stuff he found, random things like a jacket, a tea kettle, and some other stuff. Nobody wanted it so he quietly went and sat in the corner to warm up, it was nasty outside and pretty cold. As I was leaving I stopped and asked if he was hungry and wanted to have dinner across the street at burger king with me. It took a bit to convince him I was serious and he agreed. After dropping my stuff off at the car we walked across the street. As we were ordering he tried to get something really small and kept looking at me as he asked to make sure I was ok with what he asked for. I had to practically force the guy to admit that he was really hungry and order a bigger meal. He must have been starving too, that double whopper disappeared amazingly fast.
He gave me his story while we were eating. I guess he lost his job and was trying to drive to a place he heard he could get one a few states over. Something went wrong with the car's cooling system so it started over heating, it could only drive a few minutes at a time before it was in the red. He was able to limp into town so he had just been camping out in his car, moving it enough to keep the police from getting on his case, and trying to sell off some of his possessions that he had with him to get enough money to have a mechanic look at his car.
He never asked me for any money or anything, I think he was embarrassed enough that it had gotten so bad that a stranger was buying him food. The only thing he asked was if there was a place he could go to get some help. I gave him some directions to a salvation army facility (it was actually really close, but on a side street so he wouldn't have easily found it) and he said he'd head over the next day. He seemed genuinely happy when we said goodbye. I've never seen him since then (as opposed to the bums I would see over and over again in that area) so I assume he got the help he needed.
Since then I've only given food to bums, few take it but the ones that do seem very grateful. It seems to be a good way of weeding out the truly needy from the people that are just trying to make a quick buck.