I've seen a little girl being taught to beg once. She was as beautiful and clean as a young girl can be, and she was with a slightly older boy in a subway. He was leading her by the hand and showing her the ropes... what to say, how to say it, how to act... Most people there thought that it was so very charming and paid well for the feeling. All I did was to try hard as I could not to hit something or somebody, because I was watching a soul being mutilated.
i.e. I don't want to give someone money that they'll use to further destroy their life with drugs/alcohol/whatever. I don't want to be complicit in that action. If that's too 'morally' or 'judgemental' for some people, so be it.
Now, I'm more than happy to buy them some food, transportation, etc. Or even better, spend some time with them (when possible) and treat them like a real person for a few minutes.
Even if they use my money to drink themselves into oblivion, thats their choice and I'm happy that I can provide them with the means to do whatever they want with their lives.
That said, I do prefer to give to certain types of people. It can be hard to really tell how needy someone is, but generally I don't give to children (this encourages parents to not send them to school), those who occupy prime locations (main street, outside a supermerket etc. - as I've seen these being worked by groups who muscle others out of their patch - and besides they make plenty of money from others there), and those who hassle every passing stranger.