I think you are comparing RL with a fantasy world, something that is
always a dangerous thing to do. Magic, both divine and sorcerous, is not
a question, it is a fact of life on Cerilia. The deities might keep
themselves aloof of the affairs of mortals, but their servants still can
cure wounds, summon plagues or cause the harvest to be doubled. Even
when keeping aloove, they do limit their priests to a certain extend.
Blooded priests are even more dangerous with their realm spells, spells
wich can make a huge difference. The sole reason temples are not all
powerful is because of the frictious behaviour within one faith and
because there are several deities. Even then, the common populace would
hardly dare to anger the priests for fear of the priests casting the
spell 'blight'. Which in the eyes of the common peasant would be the
same as the wrath of the deity (which it actualy is, because the deity
provides the priest with the spell).
There are also individual who can destroy complete armies with only a
few waves of their hands and some arcane mumblings. Do you think,
peasants are going to argue with these kind of persons? Sure, they are
rare, but one is enough to mean the difference.
The nobles also have real signs of power, one of my PC's can let sun
beams come from her eyes and an other can see through mist as if it is
not there. The blooded are the only ones who have access to regency,
hence a regent will fail to rule if he has got no access to it. Sure the
line might pass to unblooded characters, but as you stated yourself:
power corrupts. Would the person who get the bloodline be willing to
part with it after four years in function? You might argue that regency
is not something magical, but it is on Cerilia.

I do not know in what direction Cerilean culture might go, might it have
been a real world. Most likely it would not have taken the same
direction as our Western cultures on the real world. But this is a
fantasy campaign, hence you can let it go in every direction you want it
to go.

Pieter Sleijpen