Bank says "When you play this, it is worth $1 per treasure card you have in play". It does not say what it's worth otherwise, if you don't play it. Same with Philosopher's stone; it says "When you play this, count your deck and discard... it's worth 1 per 5 cards between them." I guess technically it's still worth the same amount, but if you don't play it, you don't get to count your deck and discard and see how much it's actually worth?
Man, I guess those issues could be resolved somehow if DXV really wanted to, but why add in an extra thing?
If you need to use treasures in your hand for a card ability, then might as well use their cost, which is roughly correlated with how much the treasure would be worth if you played it and doesn't have any confusion with Bank, PStone, and other treasures which do stuff when played.