Calrin's rule would certainly make Sorcerers a lot better by comparison.
But seriously folks,
Obviously this is a game playability issue. Requiring magic users to spend hours memorizing (or prepring spells) just slows things down for everyone. I find that even the sleep and spell selection has a similar effect. Every mid/high level session I have been in, there has been almost always at least one case where we waited a full day to do something just so a spell caster could change a spell, or rememorize.
That said, for game balance and story purposes some limitations and preparation is good.
Let me give Calrin a possible explanation that might explain why memorization gets so much faster. I teach philosphy to undergrads. I tend to read the book before the semester, then I skim each chapter before each class. Each semester I use the same book, the amount of time I spend skimming gets shorter and shorter.
I think it reasonable to assume that a high level wizard (super intelligent, well educated, and mystically aware) could have a similar experience. They've almost committed all of their spells to memory, they simple flip through their books to check their memory, maybe to remind themselves of the component measurements, or specific verbal components. I know Descartes's 3rd Meditation very well, but when I teach it, I check the text really quickly to make sure I have the language down precisely. It only takes a couple seconds.
So you glance at most of your spells. Some of the more complicated ones or less familiar ones you spend more time on, and some are so familiar to you that don't even need to glance at them.
I think that makes some sense, and you can still require the long reading time to learn a new spell. That seems reasonable to me.
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BTW I played a 14th level MU in 1st ed. It took hours to memorize spells, and it irked me to no end. I eventually wrote a spell to speed memorization time (Malzerious's Memory Osmosis 4th Level, you slept on your books and memorizaed the spells while you slept),the DM was, frankly, quite relieved to see it. He rewarded character with the esteem of his peers (and the shortened memorization time).