When is Loan worth buying?
I see two benefits to this:

First, it's helping you cycle through your deck quickly, which is almost always a good thing.  Theoretically, your discard pile should contain new cards that make your deck stronger, so you want to get to shuffling them in as soon as possible.

Second, though you don't mention it, Loan gives you the option of trashing the revealed treasure instead of just discarding it.  If the treasure is a Copper (especially likely early in the game), you will probably want to trash it instead of discarding it, since this can hone down your deck.  But since Loan itself is worth a coin, it always has some value, so it won't be clogging your deck with a useless card late in the game.

Whether this is worth more than a Silver probably depends on your strategy and the other cards available.  If this were the only trashing card available, I would be tempted to buy it.

Read this blog post (and the comments) for a discussion of the statistics concerning Loan. The short version is this: If the Loan does not pull a Treasure you want to trash, it statistically does no harm to your deck, unless your deck is very small.