"This money could help this person, but it's also quite risky, given that the majority of chronically homeless people (most people panhandling) have drug addictions or other mental health issues or both. My money would be better spent on services that can help address these issues over the long term."

I think we disagree on what constitutes an asshole. Being aware of the statistical realities of a situation and operating on them is not being an asshole. If you were to say "of course I'm not giving you money, you're probably addicted to heroin" to someone's face after they ask you for change, that would make you an asshole. But that's not what I'm advocating.