A few people have thrown about the term scarcity and post-scarcity. And kind of wiffed at what that means.
Scarcity in economics means there is a need to economize something. Hence, even if something is plentiful, if it's not so plentiful and readily available that no one wants more than they already have, it is still scarce and therefore needs to be economized.
Even in a future where matter could be replicated at will, all of the following things would be scare - in other words, not enough to go around:
Unique Items - sure you could replicate Action Comics #1, but, people want the original. The history of the item matters to a collector or historian.
Labor and Services - even if there is some means of having disagreeable labor done for you, so no one has to be a garbageman, there's still specialists whose time would be scarce. For example, time with a famous musician (at a concert, meeting fans, etc), political figure (dinner with the President), or actor (paying him to agree to do a movie). The same with any particularly skilled work. Only so many people will be attended by the best surgeon, attend the best university, etc. Or time at a national park - what happens if ten million tourists visiting a park destroys its ecosystem, or too many houses by picturesque lake pollutes the lake and ruins its aesthetic appeal?
Land - More people want to live in Manhattan than in Podunk Town. And only a select few people will be able to get an place that overlooks Central Park. There must be a means of allocating those scarce locations.
So the "post-scarcity" Star Trek economy is nonsense. Some things are scarce in that universe. That society would need a means to allocate those scarce things.
Commodities in Star Trek are not scarce, thanks to replicators. But commodities are things which are essentially interchangeable. For example, crude oil is a commodity in our economies - I care how many barrels of crude I get, not which ones.
The Star Trek economy doesn't make sense, and those economies are not really post-scarcity. They still need a means to divide up those goods - which means either the Federation mandates all those things and is quite totalitarian, or the people still have to enter into a market for them.
There's a decent discussion of unavoidable scarcity in Wikipedia's article on post-scarcity. I still think even with virtually unlimited power and manufacturing capacity, there would still be scarcity (and a needs for economizing - therefore, currency) of things like unique items, locations, and services from select individuals.