I'd like to tie willpower to mana recovery, and then you could have summons with mana upkeep, so you need more willpower if you want to keep a reasonable recharge rate.
Problem is... should a wizard character really have to be spending their precious points on essence, intelligence and wisdom/willpower? Too many attributes. Maybe essence boosts should also boost mana recharge rates; so that it takes roughly a constant number of turns to replenish your mana pool; eg at 10 essence I regenerate 1 per turn, at 20 essence I get 2 per turn.
Another alternative; tie enchantment/summon upkeeps to charisma! Its a wasted stat at the moment, and it makes a ton of sense to me that strength of personality would effect how many enchantments and summons you could keep going.