I assume death extracts a toll regardless, though; and anima has be expended to return unhoused souls to a physical form - so why make the expenditure to do that when you can just abduct the targets and bring them straight to you as they are, strengths fully intact to be put to use later. I mean he's completely successful in doing that, so why go through the extra steps? Plus, you get the added bonus of implying to the rest of Azeroth: "hey, I'm powerful enough now to just take whatever and whoever I want right out from under you."
I find the criticism cliched and overwrought - I don't recall any specific arguments like this when we knew the leaders were captured months ago, so why is it a problem now when this little connective bit? I mean, is this really the hill to die on in light of the fact that we knew they were captured? There's nothing circular about it, this is just complaining about the means not meeting your expectations for reasons that I find inarticulate at best, and base grumbling at worst. Do the Mawsworn Val'kyr need some kind of elaborate backstory to make it possible for them to abduct people? Does Sylvanas herself need to appear in every case to underscore her own threat? The point about the faction leaders walking around "defenseless" is especially silly - what, do you that they're on guard 24x7 or something in the safety of their own cities? I mean all of them appear to be in the capitols, basically doing their own thing. Baine's crossing a bridge in Thunder Bluff, and Anduin is visiting his father's memorial in the safety of Stormwind - it's obvious they don't expect winged foes to come out of nowhere and ensnare them in seconds (and who really would). There's nothing idiotic about it, really. The only real point of complaint here is that they weren't afforded the chance to put up a fight (except for Tyrande), but then that's also the point of the manner of their abduction. If the Mawsworn Val'kyr gave them a moment to fight the outcomes might've been very different, but it was a lightning attack meant to take them off guard, and it was entirely successful.