Forager's another. if you're using a Forager for cash and your opponent isn't, might as well bump it up, and Mine Spoils->Silver isn't even a bad move.
Or maybe you just need more treasure. Perhaps you've drawn your whole deck, but you don't have a bunch of spare actions - you need your money to come from treasures and not have to be replenished via actions. Mine Spoils->Silver is a reasonable way - it's $1 less this turn, but you don't have to keep playing your Marauder (or Pillage) to replenish it on future turns.
Obviously the usual cases with Fairgrounds/Menagerie/HoP, maybe you need a silver for variety. Feodum. Emptying the Silver pile.
Perhaps it's a 4-player game, and the other 3 people are going heavy on spoils-gainers, whereas you aren't, so it's worth your while to lower the number of spoils in the pile to mess them up a little.
Perhaps you have a bunch of Gardens - you don't want your Spoils to go away, and Mine spoils->silver effectively increases your deck size by 1 relative to playing the Spoils, again at a cost of $1.
...or, to be more general:
1) If you want silver in your deck
2) and you don't have any other Mine targets in hand
3) and you need $1 or $2 more this turn, but don't need the full $3 from spoils
then mine silver->spoils is likely to be a good move.