This is an example of one of those glorious cross-expansion synergies. Duke strategies rely on being able to consistently hit $5, even as your deck gets stuffed with many more green cards than usual. As WanderingWinder illustrates, Horse Traders is just about the perfect card for the situation: it is a reliable method of reaching $5 (but no more), it’s quite affordable, and it doesn’t get slowed down by green cards. At the same time, Duke covers for Horse Traders’ main weakness (stuttering in more powerful decks designed to reach $8) by allowing you to score big with only $5’s.
In the simulators, this strategy crushes most Province-based strategies, and even Big Money-Mountebank. Ordinarily, a Province engine can snipe enough Duchies away from a Duke engine to settle the match with its Provinces. Horse Traders, on the other hand, can force the game into being decided by Duchies and Dukes, and in that case an engine built to reach $5 from the start is going to win in the long run.
Baron is also effective in a Duke strategy, insofar as it can consistently hit $5 in a green deck, but it’s a little more dependent on drawing Estates with the Baron. So long as you keep up your Copper buys (yes, Copper, a task made much simpler with Horse Traders’ +Buy), Horse Traders doesn’t need much support from other cards, and as a bonus, is a mild defense against many attacks as well.