If he burned the source then he's not going to be able to memorize it again in the morning; and it seems an awful waste to go through all that effort just for a one-time use at some random moment when some adventurers happen by.
I was imagining more of a demon-summoning ritual or somesuch, rather than just some kind of standard spell. Though still, even there, maybe they used some kind of ritual to give it to themself as an innate power. Again, something that took a lot of time and effort. You seem to want them to very specifically be a wizard. I think most people's point is: Sure, if the spellcaster is a wizard, make them act like a wizard. But an NPC has no reason to be a PC-class. I they're NOT a wizard, they can use magic in another way.
That's just it - if you're internally consistent with how arcane magic works, the knowledge doesn't die with him. It might die if you blow up his spellbook and-or his field notes and his was the only copy of said ritual, but that's different.
Sure. I mean, it's always better when there's a
story behind what's going on. Still, that story often won't come up in the game, so it's perfectly reasonable for any given DM to not bother coming up with it ahead of time, in particular if it's unlikely that any PC will ever find out, even if the player thinks to ask (which they often won't). Again, I
agree that a story is better. Sometimes there's so many possible stories, that
this one doesn't matter much.
Player, "How did they do that!?"
DM, "In a way that you're never going to find out."
It might not be satisfying (to
some players), but it's often going to be
true.
I mean, do you want to quit adventuring to follow in some jerk's (often forbidden) knowledge journey? To what end?
Sometimes it's just TOO MUCH WORK to figure some things out. Forbidden and Rare magical secrets seem like that sort of thing to me.