By this point we've all had our chance to shit on the Derek plot in one way or another. Be it because it doesn't make sense for the Horde to turn on Sylvanas over this as compared to her previous actions, that they still do nothing despite how against their morals it goes or what not. That's not what this topic is about though. This topic is about Daelin. Specifically, why he would have been a much better choice to raise as undead for both the Horde and Alliance plots. I am arguing from what Blizzard look to intend for the story to go, not my own preferences.
For the Horde, it gives a real reason for a turning point. Daelin is someone who's against the orcs, who died betrayed by his daughter and slain by the monsters who took his son away from him. Even conditioned into Forsaken service, Sylvanas would essentially be promoting someone who tried to exterminate both the orcs and trolls, which would give them ample reason to be pissed on top of the dubious worth Derek presents. He also makes her plan more plausible, especially if coupled with Sylvanas raising Jaina's ship, destroyed in the raid, to serve as his flagship and mass-resses Kul Tiran sailors for good measure, something that would better present how like the Lich King she's become in her necromancy. The emotional damage is the same and doesn't rely on the retarded assassination plan that's borderline out of character given Sylvanas' views on the living. It's a real military asset with a proven commander, emphasizing all the more how she doesn't care about the Horde except as a means to an end. Finally, Daelin's hate for the orcs can become a plot point during Saurfang's rebellion, giving the orcs their most emblematic nemesis again.
For the Alliance, the benefits are almost as big if not bigger. Derek is someone Jaina and her family feels bad for, but Daelin is the person around whom her arc was built on - her feelings of betrayal, Katherine's forgiveness of her daughter, as well as a reflection of the destructive mindset the Alliance is fearing it'll slip into over just bringing Sylvanas to justice. Confronting Daelin is confronting both Kul Tiras' past and Kul Tiras' future, with personal stakes in the Kul Tiran cast and ideological stakes vs Anduin and Tyrande. That Daelin is there is what their previous lenience bought, but it's also a show of how being a hate-filled revenant isn't the way to go for what follows.
Daelin is a character a lot more people have an emotional stake in, both in story and out, Derek is effectively a sad zombie to match Delaryn being a sad elf. Finally, from a logistics point of view, Daelin wasn't turned to ash, he was just stabbed, so he'd be easier to pick up. The ship graveyard can still be gone to, except it'd be a setpiece where Sylvanas/Daelin raises the fallen Kul Tiran fleet and its people as zombies. You can even have the dragon we find there as a Horde asset, continuing the Arthas parallels by likening it to Sindragosa.
Discuss.