Yes, sometimes, these 'homeless' guys ruin it for the people who really need help.
I was waiting once in my car for someone, and in the middle of the street, on the concrete barrier, was a homeless person asking for money. About 20 minutes into waiting, sitting and listening to the radio, a car comes up, picks up the 'homeless' person and another comes out to take his spot.
I was seriously stunned, and learned that if you give, it is better to give to a homeless organization than to just one guy.