As far as casters go, it seems that Mages are by far the most common lorewise. Most races have several members who practice the arcane in some form, and even races like Ogres often have magic users. However, the more you wield Arcane magic, the more you want to wield Arcane magic - it's a corrupting force, so consequently many Mages unknowingly set themselves down the path to ruin.
Becoming a Divine Magic user (Druid/Shaman/Priest/Paladin), on the other hand, is very dependent on one's faith and inner power. There are several tests of faith that one must pass in training, and it is strongly implied that these classes have much more strict alignment & personality "requirements" congruent with the deities they are following.
This COULD be part of the reason why Shamans are held in such high regard in some Horde societies compared to Mages, because not everyone can become one. On the other hand, with Arcane magic, you don't need to possess a certain set of beliefs or go through a strict training regimen, as long as you have knowledge of the spells and how to cast them.
So what do you think?