Umm, I guess you mean the minimum is two for a system that has classes? Well, that's obvious.The bare minimum one could have is 2. Caster or Non-caster...since the rogue/skill monkeys would initially fall under "non-caster" types.
Otherwise it's not quite correct: If spellcasting is a skill (or set of skills) like any other, there's no reason to differentiate between casters and non-casters. There's actually plenty of systems that work like that, some that retain the 'class' concept (typically, choice of class controls how expensive the different skills or skill groups are to purchase or advance) and some that don't.
I actually tend to prefer systems that treat spellcasting and weapon (and possibly armor) proficiencies as skills. It's one further step towards standardizing game mechanics.
As mentioned, I still prefer it if a system offers plenty of 'classes', since class-less systems tend to overwhelm beginning players with all the available options. Class-less point-buy systems also seem to encourage min-maxing behaviour, which I dislike.
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