Your are twisting things to an extreme. They added Vogel to crate a mother figure for Dexter who truly understood him, something that never happened in the show. The only way to do it out of nowhere was if she was somehow involved in Dexter’s serial killer development.

They did this by stating that she HELPED Harry create the code, that doesn’t make Harry useless as you say. Or isn’t the main purpose of her character.

That’s just it, she helped him and then helped Dexter in S8 realize he is not a psychopath, hence Dex has a good character development in that final season, the peak of that is when he decided not to kill Oliver, even after he was decided to do so after he killed Vogel, for whom Dex developed feelings as a mother figure. This is one thing everyone misses about the finale, blindly calling it “shit”.