Until the mage is somewhat leveled up, he's basically useless.
By the way, when the question was initially raised, were the boundaries between these three categories of heroes defined? Or does everyone decide for themselves?
If a hero has no combat skills, he's a mage; if he has no magical skills, not even a spellbook, he's a warrior. It seems straightforward.
But if he buys a spellbook, does he remain a warrior or transition into a battle mage? What if he learns one of the schools of magic? Or several schools (in any of these three scenarios, magic is not higher than level 2)? What if he gains wisdom? Or if a mage acquires a combat skill that isn't very strong but still exists (and doesn't affect direct damage)?