Parameters would need to be established to prevent an opposing player from simply picking up the story, (as the summoned-but-not-controlled creature is now an NPC), and playing it off as a free attack on the summoner. I certainly would. You summon an uncontrolled fiend, my turn comes around and I establish that the fiesty fiend, Rho Zyo Dun'l, is pissed at being whisked away from rummaging around in someone's larder, and looking to take it out on you.
Uncontrolled summons would have to be controlled by a neutral third-party GM, or separating the summon/control would never work in a game designed like this.
Nah, I think it should be
assumed that any creature summoned will be hostile to the summoner unless otherwise controlled. That's pretty standard for any system or mythos, after all, and for good reason.
