I am not sure if this feature is even in Phobos but anyway...
What if you were able to posses the summon for x amount of time.
Lets say me and my friend are out hunting some giants, to help us along he managed to summon a (insert monsters name here). I am then able to turn into a spirit (by some magic scroll/potion etc) and posses the monster. I then have that monsters abilities and can use them more intelligently to our advantage.
Not quite sure how it would work with supplies and mana. I mean if you had infinite supplies then the time would be quite short that you could posses the monster as you could just blast/shoot/bash away.
Just a thought