Used to play them in D&D of course, since I played way back in the day when you had magic-users and illusionists and their completely different spell lists, plus clerics and druids and their completely different spell lists. . . well, the divine source vs. the mundane source did make some sense, but not the others. And as I played on and the editions changed -- well, specialist wizards did make more sense in the later books, but there were still oppositional schools. . . .
And a lot of worlds just have magics that are just plain immiscible and why a warlock and a wizard and a sorcerer and an enchanter can't learn each other's magics. Incompatible paradigms sometimes works -- though even there I prefer a metaparadigm to fit them in together like The Chronicles of Chaos and I rather like the way the two "sciences" interact in Celestial Matters -- but often enough it's just there. For no reason. Surely, some people are more talented at some things that others, and everyone probably has to specialize somewhat, but specializing math, English, etc. doesn't actually cripple your ability to learn other things, it just cuts down on your time to do so. And there's never any overlap. Like, say, the magical equivalent of biochemistry.
Color-coded magic where a philosophy comes in gets -- particularly interesting. Especially when they try to set up antitheses etc. For one thing, everyone's magic always involves manipulating the thing they hold in the highest regard. Why doesn't anyone ever think it's too important and vital to be used as a mere toy in your hands?