In the original show there wasn't a currency but in order to have aliens who exhibit avarice and the worst part of capitalism they had to include a currency (latinum, I think it was called).
Incorrect, in the original series, there was a currency ("credits") that was explicitly referenced several times; it was also referenced in at least one, and possibly more, early TNG episodes.
Sometime in the TNG era, Roddenberry laid down an edict that money, including the "credits" that had been repeatedly referenced previously, did not exist in the federation, and so they weren't mentioned again.
> Currency was later re-introduced to enable the Ferengi race to be portrayed as greedy merchants.
I don't think that's really accurate; the Ferengi were portrated as greedy merchants focused on profit starting fairly early in TNG without direct reference to currency (gold -- not the later "gold-pressed latinum" -- was mentioned, IIRC, as an item of interest, but not in any context which implied it was used as currency); I think gold-pressed latinum as introduced as a currency in DS9 because DS9's role as commerce hub was central to the theme of the series, and having currency just made telling stories about that a lot more convenient.
It wasn't a currency in the same way that gold isn't a currency. Latinum is supposed to be a substance which cannot be replicated unlike most things so it's valued by, as you say, the avaricious.
> It wasn't a currency in the same way that gold isn't a currency.
Throughout much of history, gold in standardized sizes was a common form of currency. Gold-pressed latinum in standardized "slips", "strips", "bars", and "bricks" is exactly the same thing.
