Killing such a smart, cynical, well-rounded character as Kheylar just to give Worf an excuse to kill Duras and a little mileage on "Are you my father?" was very short-sighted. It was also short-sided not to keep Alexander on-board. They could have done the Sisko-Jake thing years before they did....but they probably already had enough flak over Wesley.
Especially since the short term effect was to make the great diplomat Picard look dumb (reprimanding Worf) and the long term effect was to make Worf a bad parent.
Edit: I'm watching "Reunion" right now and just realized if they left Kheylar alive, it would be completely unrealistic that every officer on the Enterprise would not constantly be fighting over her.