I've written and read fic where had it been said in the beginning what was going to happen it would have ruined the effect of the incident. I once wrote a fic where it was implied the whole time that someone was dead but then he wasn't. It can also work the other way around. Like a play with words, a juggle with emotions. Knowing in advance would totally ruin the element of surprise. Think about movies like The Sixth Sense, The Piano or The Crying Game. Would you really have wanted a warning in the beginning of what was going to happen? For me, if it says dark I'm prepared for anything: rape, abuse of other kind even murder or suicide.

Well, that is my very humble opinion.