One disadvantage is that the novelist can thereby give them all the right spells. Players will not blunder into a situation unprepared or inadequately prepared. Which is where the drama lies.
Especially if the spells have been useful heretofore. It is useful for magical items, too, to have a very useful one turn out to be counterproductive, but a Gamelit universe if the characters always have the right spells for their purposes, the retroactive ability to have the right ones is too obvious.
Besides, the "right ones" for the novelist are those that maximize drama. Generally by proving to be right only if you are clever enough. The earlier ones that were right for the characters maximize the impact of the desperate moments.
But some have to be missing for reasons of simple realism. If the spells are always and only wrong for moments of high drama, that shows the hand of the author too clearly, too. (Low drama is fine. They don't have to be at the moment when the lack of spells means they walk for three hours instead of using magic.)
Characters who don't have to prep spells, of course, drive the question back to the character creation. On one hand, it's more absolute; on the other, the character will never stand around kicking himself for not preparing hold monster before entering the labyrinth. Or even observing that if he had known that he would not be on the road but thrown into the labyrinth, he would have prepped a different spell set.