Wouldn't spellcasters of any sort be rare to very rare in almost every campaign setting?
And actually, shouldn't the "easy" ways to power (warlock, say) be more common than the "hard" way (wizard)?
In the historical play of DnD, every small thorp or village has someone a PC can learn magic from (to level up). Usually there is someone of around level 3-5 MU/Wizard around every place which can provide spells for the spell book of the first 3 spell levels. I mean, if you know fly and fireball then you can pretty much help the local militia beat off most attacks and animal threats.
It was when you wanted to learn beyond level 3 spells that you had to find some great college or go on a quest to find a full wizard as your mentor.
That makes a MU/Wizard present for roughly every 100 humans. Many humanoid races used to have a MU/Wizard for every 20 to 30 individuals depending on their attraction to magic.
This would hardly make them rare but more like computer programers or auto mechanics.
-- Many people might be familiar and know a few tricks.
-- You probably know one or two that you go to for your daily needs
-- You have to look up in the yellow pages for an expert for something that is out of the normal
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If organized play used rarity as a way to control class choice it would end up having a riot on its hands at the next convention. People want to play what they want to play or they'll go elsewhere.