Fool's Gold isn't a Useless Spell
There's this spell in the AD&D PHB called Fool's Gold. It turns a big stack of copper pieces into pretty good fake gold pieces for a while. But that's not all! If you want to be sure of fooling your target, you can crush an expensive gem on the fool's gold to make it doubly fooling! Why you wouldn't just offer someone the gem you're going to crush anyway is a little unclear.
Has anybody ever used a memorization slot on this? Has anyone ever used it to good effect? Fool's Gold is just one of those many practically useless spells in the PHB (Leomund's Trap, anyone? Mending?).* If you're into measuring the utility of the spells you memorize, it's hard to see why you'd chose this over Magic Missile, Charm Person, or Protection from Evil, for example.
At least, that's what I used to think. But now I'm starting to see that utility isn't the only or even the main reason those spell lists exist. Those lists show what wizards in this world can do. And if your wizard has Fool's Gold in his spell list, it's the kind of thing he/she can do too. That's a small but important part of the game of D&D.
As a DM, you can put this to good use. What happens when the Orcs that the party is trying to bribe have a dreadful distrust of Wizards, owing to their reputation for making fake treasures? If wizards have a reputation for deception in your world, it can make all kinds of interesting trouble for your party. Or maybe the kobolds in that ruined tower are helping the mage with his experiments because of that huge chest of "gold" he keeps showing them. And Fool's Gold might not be worth a memorization slot, but in a wand or scroll, it might see use. A wand of mending would be a handy and colorful item for a wizard to be toting around.
The deeper I dig into AD&D the more respect I have for Gary Gygax's game design skills. There's a lot of gold (not Fool's Gold) buried in the texts--perhaps an endless supply!
* I'm also deliberately ignoring another purpose for useless spells: to make a certain brand of player bend their brain trying to find a good use for them. I played a bard in 3rd edition, so I know whereof I speak on this.