1) Multiplayer
2) Against Mint-FG or other types of Fools Gold Games
3) ??
If by "multiplayer" you mean "multiplayer Gardens games" in particular, then I mostly agree- in non-Gardens setups not even 4p can salvage the Thief.
In addition, the existence of Thief is occasionally relevant as a cautionary tale against pursuing Chapel-Big Money or a strategy that relies on mass Harem or Venture, though against a careful opponent actually buying that Thief is unlikely to be fruitful. And in something like .000001 percent of games it may be possible to build some huge King's Court draw-your-deck with a zillion Actions engine, but there isn't any virtual treasure at all: and
maaaaaaaaaybe then you could theoretically use the Thief to lock down your opponent's buying power.
Here's an example where NinjaBus tries to build a Thief engine against my Courtyard-BM- it doesn't actually succeed, the Thief still loses, but it does have me worried for a split second, and doing things like buying Cache in response.
But, really, you'll do far better just assuming that Thief is a card that is never actually worth buying, even if that's technically not true. While I've softened my old position that it's the one card in Dominion which is always useless in every situation, I still do believe it's the game's weakest card, and in 2p at least there's no plausible competition for that slot.