I like the idea on maximum uses and limits on spells. And crafting spells is something that's been talked about A LOT for the past three years. I'm one of those who thinks that spells should be discoverable, and then craftable using the right materials. I'll try to explain what's in my head
Lets say >Urist McHunter comes across an odd stone with some engravings on it while hunting!
Now Urist McHunter was able to recognize the runestone he came across while hunting because he's Adept in Arcane magic.
>Urist McHunter drags it back home to the stockpile for later use
Later, during a little surprise, you order >Urist McWizard to use runestone on escaping Goblin Kidnapper. Goblin Kidnapper is Struck by Lightning!
Well crap, you just used your only freaking runestone, and you don't know how to craft any more.
Next time Urist McHunter comes across a runestone you've learned your lesson on conserving spells.
>New Migrants have arrived!
Among the new migrants is a Urista McLibrarian, sensing that you have a runestonowe! You build her a library, and she tells you that the newest runestone is Arrow Greater! Not that she can use it, she's just a Librarian. But she can make crappy copies of the original (as she really isn't that experienced).
>A Goblin Invasion comes to town, angry at the death of the kidnappers and starving with maternal affection for anything infantlike. They approach your fort, and you slowly target each Goblin with a low quality Arrow Greater. They don't die, but they're injured enough for your army to finish off. The end!