Might heroes are so much better than magic ones, but, a Might hero that dab into magic is so much better than one that doesn't. And, even when you play a barbarian, the mightiest of the might heroes, you'll be offered the chance to pick up one of the four school of magic once every four levels, and you will be offered the chance to pick up basic wisdom once every six levels. Even if you don't intend to use Dimension Door or Town Gate, spells like Resurrect, Mass Haste, Mass Bless, Mass Slow, Mass Shield, Mass Prayer, Mass Dispel... all of those spells have the potential of turning a defeat into a victory when used correctly, and shouldn't be ignored.
But, I cannot say how your heroes new skills are right now, I recall that in my game, Crag Hack and Yog both had, Offense, Wisdom, Air, Earth and Logistics. Gem had no Logistics but she had Wisdom, Air, Earth and Water. As for Gelu, he had Logistics, Earth and Water. I forgot what were their other skills, I recall that I tried to reroll to get more perfect skills but in the end I settled for those skills, and as soon they learned Dimension Door, Town Gate, Resurrect, and the standard buff and debuff spells, Crag Hack, Yog and Gem went on earth conquering rampages teleporting all over the place, and Gelu who, had no access to the Dimension Door spell, just stayed behind, recruiting troops and defending the castles. And, although I felt like the game screwed me over, at no point I felt like I was at a disadvantage, quite the contrary, the maps still felt like normal maps, maps where you start with low level characters and no spells. Even when you ignore the game breaking spells, being able to clear most monsters around your starting area with nothing but damage spells is tremendously broken. And so I felt the need to pump it up to Impossible difficulty to help the AI a bit.
But all in all, I know that it sucks but, due to the very nature of the carry over feature of Heroes of Might and Magic III, even if you get a couple of useless skills on your heroes, you'll still have a massive advantage over your AI opponents for the rest of the game, and this, even on the hardest difficulty level, if you were worried the screw over would affect your chances of victory.