Scribble said:
In a world where magic really does exist, and can do the things it does in D&D, why would someone ever choose not to learn how to cast spells? Even just one level... For everyday purposes some of the 0th level spells seem pretty darn handy.
It almost seems like not learning to cast spells would be the equivalent of not learning to read...
Lack of talent.
Learning to read is as easy as learning to speak, if your parents know how to read. If you don't learn to read at home, you learn to read at school, possibly before you even remember things (eg many, if not most people here don't remember how they learned to read). That happens all the time nowadays, but most commoners couldn't read, and so couldn't teach their children to read, plus it just wasn't important to them. The commoners form the large majority of the population.
Learning to read (in DnD) takes one or two skill points (for barbarians). I figure it's the same for the NPC classes that don't automatically get the ability to read.
Learning to cast spells takes a lot more effort. It's like learning to fight with a sword. In some cultures (eg aristrocratic warriors in Europe, China and Japan), you learned to fight as early as twelve. You might be training until you're sixteen to eighteen, when you go on your first campaign. If your lord isn't at war, you hire yourself out as a mercenary, not just to get paid, not even just to get glory, but just to ensure you're skilled enough to last on a battlefield.
If learning to use a sword takes up to six years to get good at it, why would one expect learning to cast spells to be easier than that?
Learning to fight is a useful skill in a setting where banditry and even rebellion are coming, and where (during a war) knights might burn down your farm, kill your son and rape your daughter and wife... yet most commoners don't become warriors, much less fighters. They're too busy trying to scratch a living out of the soil; even if offered weapons training for free, it's just not reallly available to them.
I think taking your first level in a class is actually supposed to be pretty difficult (and most people just can't do it), but you start with your 1st PC class level so you're actually capable of doing things.
Furthermore, there's the issue of talent. PCs, important NPCs and villains might have the right talents for things like spellcasting, but most of the population isn't so lucky. An NPC with Int 16 might go to a mage school and find they have absolutely no talent for arcane spellcasting even though there's nothing in the rules preventing them from becoming a mage, whereas a PC with Int 15 might have the right talent and can either start as a mage or quickly learn from another mage (multi-classing).