It's also hard to measure one's contribution in a relationship (hey, maybe they were a really good listener; or maybe they can't find time to help you move, but when your kids get sick, they'll move the world to get the best possible care for them?), so let's not go and introduce quick-and-simple rules for what definitely are relations on a case-by-case basis.

The problem that you solve is information. You don't know who's trustworthy and who isn't. If I can give a small amount of help to someone in exchange for knowing if they are trustworthy or not, that's probably worth it.

The people that reciprocate you can keep in your life and they become valuable friends. I am more than happy with the tradeoff of sometimes giving people some help that they don't reciprocate in exchange for finding the ones that will.