It can also be worth it in few other circumstances:
1. Late game where you know your draw deck is crap and it is worth it just to get back to your other good cards.
2. Late game when you want more payout from an overbuilt engine (e.g. you have steed, council room in hand, and an extra action and no cards in the draw deck/discard); this adds 8 coin to your payout and can put you into range to double or treble province when you otherwise might not be able to do so or it can give you the cash needed to pile out the last two duchies and the last two xroads or such.
3. If you are using TfB heavily, this can provide fodder to keep on using them. For instance, if you've run out of things to use your upgrades on you can grab silvers and keep turning them into engine components (e.g. smithy, farming village, Tr). Apprentice is a big one here, chugging through silvers keeps the apprentice fairly strong and late game you may want to trash a province to draw 4 new shiny silvers and end the game a turn earlier than your opponent suspects.
4. If you are getting mass thief or noble brigandage going against you. Sometimes the desperate play Kc/Thief works and you need a fresh source of payout quick. 4 silvers does that quite nicely, particularly if you are going to draw a bunch with an engine.