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.
In the latest interview, they said that the dolls require upkeep. Possibly mana per minute? Time to get your Regeneration up!
