Here is my take, Denise. Courtney Love hasn't been a musician for a long, long time, and has devolved into somewhat of a public spectacle for her substance abuse, bad relationship with her daughter and former members of Nirvana (I believe her daughter demanded and obtained the severing of her parental rights), and general trainwreck-ness. She sort of teeters in and out of public view based on her rambling, incoherent Twittering, but she has lost any sort of relevance or respect for her musicianship/coolness/whatever long ago. Even in her heyday, she seems like a poor fit for Saint Laurent. Similarly, Marilyn Manson doesn't make much music anymore and has no real cultural currency, nor does his style or presentation of self seem to gibe with the Saint Laurent brand. Picking these two makes Slimane seem bizarrely out of touch.
As I mentioned, I really like and respect Kim Gordon (she is an active, legendary musician, has dabbled in fashion design collaborations, is smart and interesting, has a strong personal style, has integrity), and so she seems like a good choice to me.