What a terrible tactic and lack of variety in Heroes 1-4. Both a warrior and a mage can learn all the spells; the only difference is that the mage gets a small bonus to spell power (although they can only cast that spell once per turn), while the warrior gets a bonus to the power of their entire army, all units, which can be 5 or 7 in a squad. And yet, a warrior with lower magic and knowledge can use magic just as effectively!
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I don't remember the cost of spells depending on the amount of strength and knowledge. +5 strength and knowledge is enough for me to cast a couple of mass spells on myself. And in general, the larger the map, the more effective the warrior is.
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+5p74, you're forgetting about specialties and starting skills. Having air or earth magic from the start can be useful, but warriors don't get that from the start.