To you it might sound like a lot of effort but for a user who is on his 1 day off for the week not having to wait twice the amount of time to log in to play his game probably means more than you or I could ever imagine. Blizzard used to be about making everyone have an excellent experience and some of the legacy features they've had for years seriously still do show what kind of commitments to that fact they have made in the past.
I think you're also over-estimating how hard it is to flag something as not-necessary for the majority of users. Especially when downloading a fresh copy of a new expansion. If you know most players aren't going to be in the final boss lair for 3 weeks but still ship the game with that content it's not unimaginable to just allow the game to download those files in the background.
I think you're also over-estimating how hard it is to flag something as not-necessary for the majority of users. Especially when downloading a fresh copy of a new expansion. If you know most players aren't going to be in the final boss lair for 3 weeks but still ship the game with that content it's not unimaginable to just allow the game to download those files in the background.