One of the things I love about playing wizards is trying to expand my spellbook as much as possible, and even if I have a few must-take spells, I try to change things up here and there, and find situational spells that'll fit the right situation.
There is also an arcane theses, Spell Substitution, that allows you to swap out a prepared but uncast spell for a different one. Very handy indeed, for the wizard with a big spellbook.
You're right, Thomas, a player not wanting to get into the minutiae of having lots of spells is better off going spontaneous.