I always thought the idea of no money was an altruistic Roddenberry-ism that is just not realistic. Holdfast explained it very eloquently, but there simply has to be some sort of value placed on the exchanges of resources and services. I think DS9 did a better job of showing the reality of currency.
I remember watching the episode where we see Sisko working in his fathers restaurant on earth. If there is no money then essentially the food is free, right? What's there to stop someone from just glutting themselves on everything with no restraint? Humans have been greedy and self serving since the dawn of man, is all that supposed to magically go away in just a few hundred years?
Excellent point. The Federation's no-money policy is not practical or realistic.
Do people work in dilithium mines because they enjoy it or to better themselves? Do people construct photon torpedoes to better themselves? If you work over 80 hours a week, it's not selfish to want to be compensated for what you've done.