How about folks from out of state who "need a few bucks for gas", and if you offer to put some money on a pump, [not] surprisingly, they decline? First off, I always like to hear about how a road trip was a jamming good idea while lacking resources to arrive at a destination.
Just as others have touched on regarding the nature of homelessness, it can be inferred that people were either unable to help themselves past a point or they chose a life of limited responsibility. Certain individuals are truly beyond the reasonable assistance of friends or passers by, and that's neither limited nor exclusive to the homeless. Stereotyping and making blanket judgments based on less than absolute determining factors is undoubtedly a moral defect... And yet, you'd be naive to ignore commonalities among types of people and attempt to give everyone your all. Therefore, we give Joe a few bucks in order to take the high road, but there will be no rides, to prevent him from staining the seats, carjacking, or killing us. Mary, on the other hand, looks like she's had a bath within the past fortnight, and she seems oh, so nice.
It's rational to consider if actions are actually going to help someone. Serving to enable degenerate habits can be worse than gambling, as you are choosing to fund choices made by others.
And, like with all financial choices, also give honest thought toward how much that money cost you. Does a $20 donation amount to half an hour of work, an hour, or more after taxes? Do you still have a good feeling about contributing?
Speaking of costs: teaching a man to fish could very well be more costly and futile than giving him a fish.
But hey, who's to say even vain efforts are a waste if they lend to senses of validation [for lacking selfishness] or spirituality. We all gotta rationalize some kinda way...
Another interesting perspective can be had by comparing this topic with how the best of intentions from parents often lead to poor results in their children, especially when support is given without regard to potential consequences. I could've posted this last bit and called it done.