There's a lot of people who ruin it for others, like those alcoholics and drug addicts, atleast we have them here a LOT in The Netherlands. Usually, if I want to help homeless people or beggars, I would just bring them to get a sandwich and some drinks, or maybe toilet stuff, instead of tossing them money and hoping they won't use it to support their habits.
kinda off-topic:
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Some guy "welcomed" me, however I think his post is hilarious xD