Another place for multiple potions is with heavy upgrading or similar. For instance, an upgrade/apothecary/hamlet/goons engine can get to the point where it draws every card and upgrading estates -> silvers -> pots gives you double apothecary buys (for more points once the hamlets run out); next turn the pots can be upgraded into upgrades (and then on to goons). Pots are a good way point in a remodel or upgrade chain for apothecary decks - they increase draw. The downside is that a typical province game will be over too quickly to make it worthwhile, goons (or other VP chips) and colonies can change that.
This brings up another point where multiple pots can be useful. Double pots will let you buy more economy each shuffle, but the number of shuffles in the game limits the utility of that economy. With a longer colony game, multiple pots can be vastly more effective. For instance, considering a possession deck. Gaining more than one possession a turn is normally not worth it in a province game, you normally hit possession when he hits his first province and it would be around his third before you'd buy a double possession - there just aren't enough points left in the game to make it competitive with buying a province yourself. In a colony game, you will often be able to hit double possession around his second colony. This leaves you with scads of points to buy (6 colonies and 8 provinces) and a lot more time left to do it. Other engines can build up in like manner. An apothecary deck strong enough to draw the entire deck for a vault, discard it, draw it again, and discard it is normally overkill in a province game. In a colony game, you have a long time to build and getting two pots can allow you to run massive overdraw for something like that. P-stone is another strong example for colonies. On weak colony boards, you can build up P-stones for a long time. Something like Goons (no villages)/Pstone may well want several pots keep buy Pstones.
To a much lesser extent, alt-VP can make double pots worth it. For instance, I could see something like Feodum/Apprentice/Golem/Grave Robber/Apothecary work with multiple potions as you will trash potions for draw and you can expand apothecaries into golems. Foedum will let the game last a long time as you snag mass silver.