Just like in every anime I ever saw (apart from Souleater) the only reason they keep around secondary charectors is for marketing, so the death of Konno Yukki does not surprise me, but nevertheless I am still depressed about it. I got attached to Yukki then they ripped my heart out, but that keeps these shows interesting. I mean would you even watch anime if there wasn't the uncertainty of whether a character you like will survive or not? So however tragic Yukki's death might have been (and it was very) we must move on eventually, otherwise you'll never be able to sleep again.