<<Seems like there's enough characters on the show, without adding a brand new cast member. If you wanted to have a regualr to replace Jadzia, it (IMHO) should have been Garak. >>
There are a couple of reasons for adding a new Dax (And none of this should be taken as a knock against Garak or Andy Robinson, both of whom we like very much.).
Without Dax we'd be down to only one regular female character (Kira) and that seemed wrong. We also wanted to keep the Dax character itself in the mix. The "Old Man" has become an integral part of the series and we didn't want to let go of that element in our show. This also goes to the main reason why we killed Jadzia in the first place: we did not like the dramatic and structural changes that we would've been forced to make in the show if her character had simply been transferred or sent off on some kind of mysterioso assignment. A couple of days ago, I said that we didn't transfer her off the station because that would've entailed transferring Worf as well and we didn't want to do that. That's true, but clearly it's not the entire reason and I probably should've expanded on that thought. I do feel that it's difficult to believe that Worf would've just waved good-bye to his wife and kept his posting on DS9, especially after "Change of Heart" -- but okay, we might've found a way around that and created some scenario that would make it work. The point is not that it was impossible, but that we didn't like it. We felt it would've been an awkward construct and that it would leave our regular characters and the audience with no sense of closure on the character.
Terry was going to be gone. She wasn't going to be popping up now and then, doing cameos on the viewscreen or dropping in to say hello, but our characters would have to act like that was always a possibility. Having her "out there" somewhere week after week, her fate unknown, while a war rages all over the Alpha Quadrant was not something that we were interested in doing. Now, you can say that it could've worked, that these problems don't seem very big to you, but we're the ones who have to actually do the work. We're the ones who have to write the scenes and adjust all the character relationships in ways we didn't believe in and didn't like. All things considered, we'd much rather bring in a new host and play the fun of our people having to adjust to her along with the audience. Having a new Dax gave us something we wanted to play as opposed to something that we saw as an unnecessary and unneeded burden on our storytelling.
And I'll say it again -- it wasn't our idea to do this in the first place. We didn't want Terry to leave the show and we said that to Terry and to the studio on more than one occasion. But $%^# happens and we had to deal with it. Jadzia is dead. We're moving on.