The fact of having 3 actresses made it a lot harder for me to empathize with her, it seemed like a totally new person each time. I understand they wanted someone older for the 3rd one, though.
I just saw her as a morality pet. Dukat changes his mind about killing her, so we at least briefly think he's an ok guy with a conscience.
On the other hand, I almost got the feeling he did it to try to get into Kira's pants. If he'd been with someone else, I think he'd have killed her (especially since it would be a scandal and his family would leave him). So I kind of feel like, rather than this act showing the audience Dukat's a nice guy, it's a manipulative act to show Kira he's a nice guy. I see her as a plot device, but I like that she exists in the series. It's not that I think Dukat is that horrible, I just think he looks at the world in a very skewed way. And he had a lot to lose by not killing her. The one thing that might have tipped the odds in his favor when it came to Kira would be saving Ziyal, and he seemed to think this might set the stage for the three of them playing house and living happily ever after.
There's also the fact that Dukat's really into Bajoran women and she's sort of a token of that, as others have mentioned.
And also the way that she adores him and he's very full of himself. He thinks everyone should adore him, he thinks there should be statues of him on Bajor, so of course he appreciates that at least one person looks up to him. His need for adoration is treated pretty heavy-handedly in episode Covenant.