Sparrow
Type: Action
Cost: $4
Choose one:
+2 Cards or +1 VP.
You may play a Sparrow from your hand.
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When you buy this, put all Sparrows from your discard pile on the top of your deck.
Nonterminal unconditional VP tokens have an obvious Golden Deck abuse potential. Sure, it is no cantrip but with some draw power you can get more than 5VPs per turn.
It is easier to buy a Duchy than to play a Sparrow three times.
True that. But it is harder to gain a Duchy and (the equivalent of) a Laboratory than it is to play 3 Sparrows. If you compare green and VP tokens, you have to take into account that the former takes space in your deck whereas the latter does not.
Plunder / Monument in Capitalism / Groundskeeper are a way more abusable.
I agree that some cards like Groundskeeper or, probably the worst culprit, Goons can be very centralizing. But all cards and landscape cards that produce VP tokens either feature a limited supply of them (many Landmarks, Farmer's Market) or lead the game towards the end, either because they produce Coins (all Prosperity cards) or because they require gaining (directly like Groundskeeper or indirectly because they require stuff to trash like Tomb and Temple).
I don't think that one should violate this very design principle, it is obviously not accidental.
Back to the actual Golden deck potential of Sparrow, while I don't think that the card is centralizing in many Kingdoms, the Golden deck is easy to set up. For example a deck that consists of 9 Sparrows (theoretically obviously not possible, you do at least need a gainer) yields 7VPs per turn (play 2 Sparrows to draw, 7 for the VPs).