I like the idea of opening said "portals" to let demons run through that are hostile to both the summoner and enemies.
Could be something similar as the Shaman in Shadowrun ; when opening the portal, there's an AP cost / turn (that increases each turn) that the player has to spend on the said portal to keep control of it (and its minions).
I wouldn't enjoy using let's say a source skill with 3-4 ap cost to open a portal and the said demon instantly attacks and disables my glass canon character the next turn...
