If you have a deck that is designed to do 16$ double Provinces, and your deck contains 16$, and then it trips over itself and you draw a 3$ (or probably an even better example is that you've just put the finishing touches on it earlier in the reshuffle, and the last five cards in your deck only produce 3$ to spend towards your engine), you probably shouldn't buy a Silver because an 18$ megaturn is no different from a 16$ megaturn, even if you have extra +buys floating around in it. It just makes it easier to draw hands that don't actually explode into a megaturn.
The silver or nothing question is a question that kind of gets glossed over a lot, oftentimes Village or something is at the 3$ price point and you just buy that and you don't have to have the Silver or nothing question. If there is no 2-3$ engine card available, you're less likely to be running a Silver-hatingly-smooth engine deck in the first place. If Embassy had a "may" in its on buy effect, or something like that, you would get a lot of interesting situations where declining Silver is a tough call, and something a skilled player will actually do.
Declined Jester Silvers week before last in a KC engine because KC, I think my less experienced opponent thought I was insane. I was insane for buying KC over Plat in that no-trash board, actually, but not insane to decline Silver once I already had 3 KCs.