Adding a new character is not the way I would have liked the series to address the gender imbalance caused by Farrell leaving. Nevertheless, I think Ezri brought unique, positive assets to the series.
First, it gave a basis to dive deeply into issues related to separation, divorce, and mourning. Ezri and Worf come as two people who once had a deep relationship, but who have trouble moving on. There is a tragic realism that comes to the fore when they cannot find the means to communicate, or when there are residues of passion that can lead to nowhere ... other than disappointment. Divorce isn't part of Star Trek, only happening in backstories or off screen, and not addressing it in some way, even by a contrivance of science fiction, is a pity.
Second, the character allows a fresh look at psychological integration. Ezri isn't just a young person trying to fit into a new job. She bears the weight of pre-established relationships and expectations, all of which are difficult to manage. It perhaps happened too quickly, but Ezri progressed from someone who might have thrown her opportunities away because of her new joining into someone for whom it is merely one aspect of a more complex personality. Indeed, she is a counterpoint to Dukat, the man who broke down because he could not balance out being a family man, a national hero, a cunning bureaucrat, a generous man, a ruthless dictator, and to the end, a narcissist.
I suspect that Ezri made people feel uncomfortable. Good. It brought it more of the qualities that defined DS9.
