There are plenty of instances (in addition to Doctor) where you gain new information after buying a card which influences whether or not you would want to spend more tokens to buy a second card.
* you buy a Ruin, revealing the next one
* you buy a Knight, revealing the next one
* an on-buy effect reveals new information, such as a Noble Brigand discarding two of your opponents key cards, signaling that it will likely be safe to buy the penultimate Province rather than the last Duchy.
* an on-gain effect reveals new information, e.g. you buy an IGG and your opponent trashes the Curse with a Watchtower. Now you'd want to chip in an extra coin token to buy a Gold rather than a second IGG.
* you buy Rats or Cultist, trashing it with a Watchtower, and drawing new cards. Maybe one is a Hunting Grounds, making you want to spend extra coin tokens on a Farmland to gain both a Province and Duchy!