10/23/2013 - These cards are silver-bordered not because they're silly, but because they're not printable for tournament Magic. They utilize your opponent's sideboard, which is not public information in a tournament, and isn't even a defined game term outside of tournament rules and Jester's Sombrero. Take a look.
The idea is that Unwanted cards aren't good enough to play on their own, and so players who open them in Limited will leave them in their sideboard. If you're willing to take the chance that your opponent opened some of these, you can run it and have it powered up to the point of being decent or even good as a result.
The trick with these cards is finding an effect that's distinctly unappealing on its own, but somewhere between playable and decent when your opponent has a copy festering in her sideboard, and good to impressive when they've got several, depending on how the effect lines up with your deck's strategy.
It probably isn't impossible to make 'sideboard' a term black-bordered cards could reference, but regardless, we don't want players searching each other's sideboards (that defeats a fair portion of the point of a sideboard) and you can't expect all players to be honest about what is or isn't in their sideboard when asked. We certainly don't want to have to call a judge every time you cast an Unwanted card at a Sealed event.
And since these aren't particularly funny, they don't belong in an Un-set either. So why share them?
It's the weird, crazy ideas that leads to the surprising, genius ideas. Sometimes directly, more often through a long chain of complicated and/or subconscious connections. Point is, don't hide your crazy. It may be trash to most, but if it's treasure to one person, it was worthwhile.