and the lifetime would be limited :
1 round only + another round for each 15 INT
(or 15 ATT for the iron fist version)
and still you will pay for summon + for each round (where each subsequent round will cost double so much money - what would make it much MORE tricky on impossible
)
And yep - the stack of reserve units may pop up next round after summon (like demon gate)
What makes sense - they are not part of your army - they are back up army - so it takes some real time till they can marsh in the battlefield... (or float/fly, whatever)