Without the serial kidnapper the cop wouldn't want to protect her, and without the blackmailer the CEO would not want her to protect him.  Without these two elements there is no need for her to be strong, no need for any of the characters to interact.  What could this story be without the crime element?

Actually without the killer and blackmailer the story might be pretty bad.

the drama actually does a good job of transitioning between the different genres. 
the kidnapper part is necessary for bong soon's character development and for her to learn how to use her strength to help those around her.

i think that the crime and mystery category def adds a little spice into this drama. it is sure one spicy sausage

I think it can be argued both ways, it would fit better if the writing was a bit more solid. The kidnapping stuff in episodes 7 and 8, especially 7, felt like poorly cut in add-ons in the show.

I think it's necessary. It definitely adds a depth to it... I went in expecting it to be a light romcom like WLFKBJ but was super pleasantly surprised when we got the crime element. It's the catalyst for Bong Soon realizing she wants to embrace her powers and use them for good and truly help people... which I don't think would have been the case if there had just been the blackmailing story (which I definitely don't think is finished yet, I mean we're only have way through... it was all too simple). Also I think it gives Gook Du more purpose as a character so he has slightly more plot than just being a romantic second lead... but also it was the catalyst for him realizing his feeling for Bong Soon as well so.

I personally think they're doing a great job switching dramas. The kidnapper creeps me out in exactly the way a kidnapper storyline like this should. I'm interested to see where they take everything.

Yeah, the kidnapper is definitely needed for Do Bong Soon's growth to an actual hero.  Right now she's fully capable of handling bullies and gangsters, but as we have seen when it comes to true evil her emotions tend to take over and she becomes weak.  In order to confront the kidnapper she's going to have become the hero she so wants to create in her games.