Getting back to Kasidy, the writers just didn't seem capable of conceiving of her as a multidimensional character who might be in love with Ben but also have a dedication to the Maquis. They cheapened her character and reduced her to just an extension of Ben. But as with Quark, they also had a habit of dropping the ball on a lot of characters on the show.
I'll have to say I disagree on this one. She was more than just an extension of Ben and he was more than just an extension of her. Wasn't her smuggling cargo to the Maquis a demonstration of her dedication to them--to the point where she even went to prison for it? That doesn't sound like someone that's just following Ben around as he leads her by the nose to me, but that's just me.
And I don't think they dropped the ball with Quark at all. I think you'd be hard pressed to find many shows sci-fi and non-sci-fi that handle recurring, minor, and secondary characters as well as DS9 did. They are out there, but they are as plentiful as needles in haystacks, imo.