There are one-trick pony specialists. There are book-wizards who can cast anything in the SRD or more if they have access to a good library, pass the Spellcraft checks and soak the damage. And somewhere in between are druids and hedge-wizards, who have a harder time casting spells (+10 to Spellcraft DCs without a book) but have access to 20-30 spells per spell level (again, at a cost in time and HP).

Who are these people? Where do they come from?

Specialist casters have some obsession, some psychologically unhealthy (in our world) fascination with death-and-undeath, or fire, or manipulating people (narrow enchanters), or some animal spirit, or trees, or frost-and-cold-and-winter, combined with some level of magical talent.

So I think the D&D divide between arcane and divine casters is replaced here with generalist and specialist casters. Maybe I should switch those terms to broad and narrow spellcasters. Broad spellcasters use established formulae for predictable effects, codified in books. Their thaum gets tied up in book-magic. NArrow spellcasters tap into the power of their strange psychologies. Their thaum gets tied into their theme.

You grow up in a society, and you're expected to pull your weight and justify the food you eat (especially if you're not providing the food). If you're in a society well-organized enough to have book-wizards and social casters, you're going to be pushed away from being one of those narrow spellcasters--you're a lot more good to the family and the town and the kingdom and everyone in general as a book-wizard or as a pantheon priest (social caster, bard) with a flavor-feat (bonus to spells with X descriptor, Improved Familiar etc) than you are spending all day staring at a sacred (to you and no one else) fire or a bunch of (pilfered) skulls or running around the woods communing with the spirit of the wolf. (Although that guy is at least hunting, and probably bringing some food home to share.)

So narrow spellcasters are found either at the apex of wealthy societies, where you have aristocrats who don't have to justify how they fritter away their time--or savage societies where having a narrow spellcaster (who can at least do some cantrip healing, or Channel Energy for healing) is better than nothing.

In between, you have societies well organized enough to support hedge-wizards and druids, but not rich enough to afford a full wizards' guild (expensive set of spellbooks, at least one wizard for each school, a dedicated building or section of a complex). This is where you tend to find your druids and hedge-wizards. These folks are an asset to a community, so people will make sure they have food and firewood and houses and things so that they have time to druid and hedge-wizard. When Timmy falls down the well, someone who can pop a Cure Light Wounds and brew up a big batch of herbal ointment and mildly magical broth (DC 15 Heal check) is a pretty good person to have around. (Or, to use a sleep spell as an epidural, if you figure out a way around the duration...)