Heroes 5.5 WarGame Edition - Tactical Edition.

Download the spellbook just to get the ability to place a mark early? That's a very strange decision, considering that the spellbook is taken for early-game powerful/top-tier spells, not for using a free (in terms of mana) ability.

Can you make the AI use summons, because without it, balance in 70% of single-player and multiplayer games with AI is useless?

No, for this, you need to edit the AI's "brain" at a deep level (in the game's code), which is needed for playing out summons. And not just change something in the game's resources.

The perk is not useless in the realities of Heroes VI, because it's for it that you get the elemental summoning spell in the spellbook. That is, you don't have to go further into the final Fire Mages, but you will still have the spell.