JD, for people like you, a reassurance ruins the story because you read to find out what happens. That reminds me of my husband, who feels cheated if someone tells him which team wins when he hasn't seen his videotape of the game yet.
OTOH, people like me read to find out how it happens. We're the ones who watch the videotape after we know who won because we want to find out how our team managed to snatch a win away from the other guys. But if I know we lost, I won't watch. I don't need to live through a depressing outcome--in a game or in a story.
However, knowing the outcome is good doesn't seem to stop me from being tense or frightened or heartbroken while I read through it, so I don't think authors should assume that a WHAM warning or a reassurance causes loss of suspense or dramatic impact for all readers. Maybe some of us stress over stories more than others do, and we need to know that things will eventually work out okay to bring the suspense down to a manageable level.
In any case, to accommodate readers like me who take depressing stories too much to heart as well as readers who feel that WHAM warnings or reassurances cause too much loss of suspense, a WHAM thread seems like a reasonable compromise.
