I ran some simulations of this case. In my sims, Merchants did not significantly help. Silvers were often remodeled to other cards, so Merchants could not reliably get a Silver +x$ when drawing full deck. This led to other $3 cards generally being more useful. If strategy is changed to always keep at least 1 Silver in the deck, then buying Merchants is much better than not buying. However, forcing always 1 Silver to remain in deck hurts you more than the benefit of the Merchants. There are some corner cases of hand combinations where Merchants could be useful, but they were too rare to increase the overall win rate significantly.