Maybe...a player should pay not only for summon, but for each turn they fight at your side
And you must press "Pay gold" button each turn or...if they aren't dead..they'll become very-very maaaad
And I've already said...it can be also weakened by giving this ability to all enemy heroes and by limit to 2nd+ round to summon (time to ready troops to fight). Or maybe this summon should work not instantly, but on the next turn. So I think that constructive criticism is much better than saying "No!"