Well as the name says they are summoned. Which means they probably can be unsommend to. Maybe a summoning duration applies. A duration that is based on your level or something like that.
A summoning doll might have come in handy for beating Fergus for example. The extra hand just might make the difference in some occasions.
I wonder if you can summon barrels.
I carried a barrel or two in my inventory in the demo. When I fought Fergus, I took out my barrel and positioned my characters to the left and right of the torture master. Then I kept throwing the barrel untill I killed him... Which was six times or so.
With the ghosts, I just put a barrel before the door, opened it and threw it in to the ghost. 3 throws and goodbye ghost <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />.