When you kill a wizard and loot his spellbook (assuming it is not trapped in anyways), not only are you ridding the world of a wizard with all of his or her knowledge but those spells are given to another wizard and not a new one. Does this make magic rare enough? I don't mind sending my wizard on a quest but what hoops do most of you DMs make your players jump for new spells.
I use all the methods -- killing a wizard for his spell book, scrolls (looted, begged, borrowed, or stolen), and pure luck. But, I also assume that just because you kill a wizard and take his spellbook doesn't mean that you make a simple roll when you're back in town (or resting in the dungeon) to add the spell to your "learned" list. That roll, for my game, is the _potential_ to learn the spell. That just means your wizard has studied the spell and understands the methods and materials. Actually knowing how to perform those methods and possessing those materials is where the real hoops kick in. I'm fully embracing the "every spell is different from every other spell" concept.
And on the wizard's quest for magical power: This is what I love about the game -- Clerics and Wizards become the real drivers of adventurers and adventures. Thieves would rather stay in town (where the MONEY is!) and Fighters like to hang around the barracks and town (where the ALE and PROSTITUTES are). Wizards and Clerics have a built-in reason for researching, organizing and even funding expeditions with those other "shiftless classes" as servants and hirelings...or rather "partners." Wizards to empower themselves and Clerics to empower their god (and thus themselves). It's an elegant solution to an age-old RPG problem... love it.