Their plot in The Siege is one of my favorites in the whole series. They really were on par with O'Brien/Bashir for a while, it's a shame it all fell apart.
I think one of the reasons it fell apart was because Kira looked so out of place doing things that Dax wanted to do. I mean that scene in Way of the Warrior where they were in the holodeck and then first meet Worf, that was close to an embarrassing scene for Kira. Then you had Worf and Dax pair up and the rest is history. It is a shame we didn't see more of Kira and Dax though. I do know they both looked for each other in terms of advice, but that was pretty much it as the series went on.
I think it touches, indirectly, on an issue that was present in all of the modern Trek series: imbalance of the sexes. Each series had no more than two female leads at a time (except for the first 20 episodes or so of TNG). In fact, not once, but twice, when a female character left the show, another female came in to replace her. [EDIT: Forgot about B'Elanna, so my argument isn't as strong as I first thought.] Logistically, that kind of arrangement will make it more likely that the writers will try to pair up two otherwise not-very-compatible female characters.
I mean, it makes senes from a 20th-century real-world perspective: two women in a primarily-male group will probably gravitate towards each other as friends, just as two men in a primarily-female group will probably do the same. But, just like in the real-world, a friendship born out of circumstance is less likely to thrive than one born out of compatibility.
We see it a bit in Troi and Crusher, as well. (I'm thinking of their "girl-talk" in "The Host," when they're at the spa [!!] together.) Generally speaking, we don't see much of a friendship between the two of them, except when it's expedient to the plot. (Of course, that could be said of many relationships in TNG, but I digress.)
In any case, I'm not trying to impugn the writers' motives here. In their position, I probably would've created the exact same cast. But I think it does play into the Kira-Jadzia relationship, its rise and demise.
"Tacking Into The Wind" is f'bleeping epic. Love that episode. Along with "The Dogs of War," best two episodes of the Final Chapter (including WYLB).
I've said before how I think by the end of season 7, the war had sorta tumbled out of everyone's hands (in-universe). At the beginning of S6, there's a bit of a problem-of-the-week attitude about the war. No one's really thinking about what it'll be like after the war (partly because no one knew if there was going to be anything after the war). But now? Now people have realized that there's a good chance they'll come out of this alive, one way or another, and they'll have to deal with the consequences, whoever wins the war. Like any trauma, it's actually two-fold: there's the trauma itself, and then there's the trauma of realizing for the first time the lifelong ramifications of that trauma.
Killing Gowron was a manifestation of one of those life-long ramifications of the war. I think Worf and Sisko knew the war would end within the year, one way or the other, but they also knew that killing Gowron would have consequences for years or decades to come. There's a particular weightiness to it that I don't think we see earlier in the war. (Like, "dying is easy; living is hard.")
In that way, "Tacking Into The Wind" captures very well, I think, just how tired everyone has gotten of the war. (Not unlike "Paper Moon" captured how Nog's eagerness had been blown away.)