Well, how COMMON it is in D&D is actually GM's choice. A *PC* can choose to be a mage, but then, it's assuming that the PC has the talent, period, and that spell selection is the only constraint guided by INT. You've got to be bright enough to cast it no matter what your talent.

However, a GM could (and I do) rule that in the GENERAL population the percentage with the talent to become a full-blown wizard (as opposed to learning simple cleaning cantrips) is very, very low.

Similarly, how many people the gods choose to actually bestow their powers upon could be very small.

The Balanced Sword trilogy begins with "Fall of Saints" -- I have that one done. I posted some sections of it in my LJ some time ago.