A second Potion is, of course, much more valuable if you're going to get +buy as part of your deck. And ESPECIALLY valuable if Vineyard is on the board and you're going to get +buy. Like, a University/Wharf deck is often going to generate buys in excess of the pure money that you'll generate, and will also probably draw both of your Potions frequently at the end of the game -- so the ability to suddenly spike the Vineyards pile is really valuable.
That's quite a special case (Potion that you're going to use for other reasons, +buy, Vineyards), but it's really powerful when it comes up.