You're right. Having a secret stash of cash is very different than pre-nups. Keeping secrets (other than "well...yeah, maybe your butt does look a little fat in those jeans") is no way to make a relationship work.
A different problem we face but I would be interested in what others think.
Say, the bride is from a well to do family, not rich, just monied. Groom is from no money at all.
Does Bride's parent leave money to the pair or to the daughter. I think, given all my daughters are married to men I love, that I would leave money to the couple. I know they all share, have joint accounts, are decent people. Others tell me I am nuts. But somehow I see them as "couples" rather than my child married to someone. It just feels wrong to treat the husbands as people who aren't related to me even though I consider them sons.