Part of the reason the ones who realize they have an 'unnatural' talent stay quiet about it is because, in that world, it is considered unnatural. There are religions that will hunt you down and kill you if they can (if they found out, or sometimes even just suspected someone of magic use). They think you are in league with demons and such.
I find this way of "controlling" magic-users to be way unfair for the magic-users, and not in the way you think. How on Earth can there be 20% of the population able to do magic - and I am assuming that a certain percentage of that number are full blown magicians, sorcerers, etc... as well as the semis - and they allow such a thing to come to pass? Where they all just absent the day that came up to vote? Can no two or more of them stay within 100' of each other so no unions or cabals or guilds could form? People able to toss fireballs and lightning bolts around don't tend to stay under anyone's thumb for any real length of time, specially if they can come together in any numbers - and would have non-mages working for them, as well.
The only way I see something like this happening for any long period (it could go for a while, but eventually, just like with technology, the oppressors will be overcome) is if the god (or gods) demanding the deaths of mages have an active hand in this and they are just way too powerful for any mage to deal with. Otherwise, you will eventually get some powerful mages who would put together forces of their own and they would go to crush the priests. While they may not win at first, eventually they will, that is the nature of the beast. So, if you have this as part of your world, I assume it is just a certain period of its history. (Though, of course, a GM may do as they wish with
their world, it just isn't "realistic" to assume a static society based upon the domination and/or annihilation of around 20% of its population - not for long anyway.)
I guess I just prefer things to be more organic and not so forced or artificial, as that seems to be to me. Also remember, that religions to us seem to be these massive organizations that controls millions of peoples lives, and it is natural to think such would be the case in a fantasy world with actual "proof" of gods. But, I do not think we should be using Christianity or Islam or Judaism as templates for fantasy religions, instead look to the pantheon religions like the Greek and Norse Gods. They held sway over much smaller groups of people, not even known a few hundred miles away - the gods of
those people probably didn't like any proselytizing of other gods in "their" lands. In fact, I think the vast majority of gods would be too busy dealing with the conflicts between each other and their clergies to do much about the mages in the world. (Of course, all of this is predicated upon having a pantheon of gods in the first place.
)
One other thing: with all the dang monsters in the world, why would a society willingly decide to throw away what is likely their greatest defense? Sounds like taking away your soldiers weapons when you know the enemy is outside the wire. (But I guess we can chalk this up to human - or near-human -personalities overriding common sense, it happens all the time.)
