I think the quote "What they don't know can't hurt them" is complete bull crap. Yea, sure, the truth hurts, but if you don't know the truth, you won't know what the right thing to do is. You might think it's the right thing, but it's not, because you don't understand what's really going on.

For example: a husband could be cheating on his wife. Wife doesn't know, but still the relationship between them is iffy. Husband doesn't plan on divorce, but continues affair. Wife might want to stay with husband (not knowing what he's doing behind her back), while instead she could be divorcing that ass and meeting a new guy who's actually loyal, and spending the rest of her life in complete happiness.