From what i've seen, all I can simply say is the the Blood Pact is just another damn plot device used to allow certain situations to happen, and just keep the plot moving forward and somewhat interesting. It's what drives Daein to fight against Ike, it's what got Ashnard in power, also kinda adding onto his evil and nutjob persona, risking his life and all of that jazz, and it's the reason Naesala seems like a bad-ass anti her for most of GC/Wii era. That's all there is too it, it's a plot device to help develop interesting lore, stories, and just keep stuff moving. Doesn't seem like they put much other thought into it.