The only time you might get lucky with experience columns is when playing as a Necromancer, and I once leveled up almost all 8 heroes.
Level 29, 16, 14, 11, 9, 7, 7, 4... there was a lot of excitement, as I managed to do it within 1 game month.
Native heroes, specialists in troops, are always bought, even if there's nothing else to do... is it worth mentioning the usefulness of such heroes as Ilaina, Ingvar, Hafiz, Dugald, Carly, Ossir, etc. + engineers.