As we discussed in another thread recently, I would have preferred if they had modeled this Republic of Pirates more in view of the barbary corsairs of the Mediterranean.
However, regarding the comments that the pirates should not be considered a real "civ", I would like to point out that we also had Venice in Civ V, which also should not have counted as a proper "civ" in the traditional sense of the series.
I also think that a pirate civ actually works better within the context of the civ switching in Civ VII than it would have in earlier games. Since the Republic of Pirates is one choice in one age, it is not really a civ in itself, but more like an episode in the history of your civ. So your civ basically becomes a rouge pirate state for one age, before becoming "civilized" again in the next age.
And even if this Republic of Pirates actually only existed for 12 years and did not become a proper "nation" in reality, Edward Teach and the other "governors" of this republic propably envisioned it as one day becoming one. Just because they did not succeed in real life, this does not mean that they cannot succeed in an alternate version of history in a game like Civ. After all, isn't this game series about alternative "what-if"-histories? So what if this Republic of Pirates had been more successful and existed for more than 12 years? Why shouldn't you be able to play out something like this in a game of Civ?
From this point of view, I see the Republic of Pirates as an addition to the game not a detraction.