The bonus of the Heretic lets them ignore all Wards when casting spells.
The Heretic is most likely to learn Life magic and Chaos magic and fairly likely to learn the Order magic, Death magic and Nature magic skills. Non-magic skills can also be learned when leveling up, but have less chance of appearing for selection.
But there's only one creature with Life Ward in the game: the Devil, and the only creature with Chaos Ward is the Titan. So unless you're fighting large amounts of either of those creatures, or facing a Ward-casting Life magic-user, the bonus is wasted. Adding more magic schools to their arsenal, to take advantage of the bonus, would turn the Heretic into an Archmage, costing them their bonus.
The combination of healing magic and damaging spells is still great in combat, however.
|Heroes of Might and Magic IV classes|
|Standard hero classes||Knight - Death Knight - Lord - Thief - Archer - Barbarian - Priest - Necromancer - Mage - Sorcerer - Druid|
|Advanced classes||Archmage - Assassin - Bard - Battle mage - Beast lord - Beastmaster - Cardinal - Crusader - Dark Lord - Dark Priest - Demonologist - Enchanter - Field marshal - Fire diviner - Fireguard - General - Guildmaster - Heretic - Illusionist - Lich - Lord commander - Monk - Ninja - Paladin - Prophet - Pyromancer - Ranger - Reaver - Seer - Shadow mage - Summoner - Warden - Warlock - Warlord - Witch king - Wizard - Wizard King|