You're right. In this case, it's easy to see that helping was the right thing to do. There are many possible scenarios where offering help is good. There are also many situations where offering help makes you a mark, or, in the worst case, threatens the safety of yourself or people you care about. That's why I say it's highly situational. Many variables to take into account.

that's funny as fuck (not for the person who lost their wheels)

A three wheeled car?

The easy litmus test is to offer to buy them the thing they want.

This is very true.

When I worked downtown and being asked for money was common, my rule was to ask them what it was for and then offer to buy it for them. More often than not, they'd decline or walk away when you made it clear you weren't giving money but would buy what they needed... food, bus tickets, gas, whatever. A couple times I returned with their requested food and they were gone, but I just gave it to someone else that always was really happy to get it. Most that took me up on the offer were genuinely grateful and so expressive in their appreciation, and I got a few entertaining stories out of it too.

Yes, in this case helping was clearly right. All I'm against is the belief that you should always offer help. In many cases (not all, or even most), that just makes you a mark. Though if you personally would rather risk getting taken advantage of than risk denying someone in genuine need help, that's up to you.