Seriously, this can net you 5 cards out of one buy. Pick up a Journeyman with each BV, and you just bought an engine. Later, you can pop open the BV and get two Duchies out of it, if you're running low on points.
In terms of general SM strategy, the overpay in general would seem to be excellent with Vineyards - every time you buy a SM, your Vineyards are each worth 1 point more. Awesomeness.
SMs also seem to be great for transitional strategies. In the game with BV/JM, I started off with an Advisor/Silver engine, with a Forager for some trashing. Once I started getting $6-$8, I started pumping in the BVs and JMs, with a couple MGs. At this point, Advisor became more of a liability (though it's very nice as an opener, and with BM), so I SM'd them into double Silvers. So if your opening strategy is different form your late game strategy, SM is great for both getting you the later cards, and cleaning up the earlier cards.