ECQ: But you won't have 15 otherwise identical cards most of the time in an actual game, that is my point. I get the basic math, adding a duchy decreases money density of whichever stack it is in. That being said, the stacks are rarely equal. If one of them tends to contain more money, then you want to play that deck more. When you've bought a province, you know you have 8 coin, and very likely 8 dense coin, in the discard. Unless you have a low variance BM strategy going on (e.g. BM/envoy), you will find that cycling can be a net benefit with venture.
If I understand your question rightly, I have something around a 30-40 card deck and I have a province buying hand of gold/gold/chancellor. In that case I am going to want to know the composition of the deck, but generally in BM decks 8 coin should be at the upper end of your variance distribution. In that case I have a better than average hand going into the discard and average hands already in the discard and average hands in the draw deck - chancellor all the way. Now for something where I have low variance, sure don't chancellor. The easiest example is a hand of
I get that venture doesn't help your next hand inately, however it gets you closer to a reshuffle. Think about it this way, if you've just seen all four of your golds go into a province buy, would you rather wait one turn or two turns to see it again?
FTL:
Yes I know, you get to play all your goodies every time you reshuffle in a stash/chancellor deck. Suppose I have a deck of 5 ventures, two plats, and a forge. Most reshuffles, this deck will ALSO play all its big treasures and buy a Colony. If I have a surplus venture, of course I want to cycle and get back to the reshuffle and the plats. Stash chancellor is merely an extreme example of this cycling back to some huge payday (for this discussion we can think of the 4 stashes as simply one giant card that reads "place 3 cards back on top of the deck, gain a province" - cycling back to that card as often as possible is what you want), all ventures/plats is merely an extreme example of venture cycling/sifting. Now do you want to cycle past your big payout cards (e.g. witch, expand, council room, etc.)? No. But venture cycling, like HP and farming village rarely skips the goodies.
At the end of the day we can look back to money distribution. In a normal BM deck a green card has a coin value of 0 and a space of 1. In a venture deck the coin value remains 0 but its space is <1. For some percentage of the time we will draw the green with the venture whereas all the other cards in a BM deck will always have to be drawn in place of something else. Venture is more green tolerant no matter what terms we use to describe the deck, you simply don't see green as often and can have more of it in your deck than would otherwise be sustainible. The extreme cases make this plain, the light trashing options make this less evident, but it still is there.
Frankly, I'm still at a loss here as to why BB maintains that venture cycling is a bad thing. It only cycles past worthless cards and when it might actually matter (at the end of the game when you hunting the last few provinces/duchies), it helps.