There's a whole lotta stuff we don't know about Edo Tensei, it's inner workings, full capabilities, limitations. Kishi owes us one hell of an explanation when the time comes, or I'm gonna be pissed. Hopefully we'll be able to discern better as we see them fight, and if he shows us a new Edo Tensei of Kisame or Minato in the works.
As for your question, maybe they get unsummoned, or just stand there like idiots once the task given to them is completed. I don't think even Oro understood the full capacity of the Edo Tensei jutsu, he thought there was no way to take them out, he was unaware of things like Dead demon seal, the rinnegan, or even the sword of totskano, all those jutsus being possessed by powerful shinobis. There should be other ways of releasing the souls from their Edo bondage as well. We need more info on the Fuda tags, what level of control the Edo Tensei souls have on the bodies, if they can will themselves to overcome such control or not.
Kishi also owes us a more detailed explanation of the yin/yang concepts, that kakashi was gonna explain to Naruto when yamato intervened. I have a feeling this'll all be related to the Edo Tensei jutsu explanation as well.
One thing is for sure, the Edo Tensei does have weaknesses and are not the full power that would've been displayed if the shinobi was alive, that's due to many reasons such as them not having a clear thinking mind which they could use to execute effective battle strategies, they lack the "will of fire" which is when a shinobi is strongest when he has some1 to protect or the "hatred" when a shinobi's desire for revenge is so great, he endures, and most importantly the chakra levels, especially for the ones that are special physical energies like Nagato and senju/uzumakis. Will their physical energies be restored by the lowly sacrificial bodies providing the yang chakra? I doubt it, it would hinder them greatly. The Edo Tensei bodies are said to keep fighting forever, however their jutsus still require chakra, so do they tire out, Kishi owes us the chakra explanation for Edo bodies as well.
Anyways, I think it'll come in due time, I have faith in the mangaka. Hopefully soon after Kisame is taken care of, yamato decides to help Naruto out with the yin/yang concepts and suiton training, while at the same time, Kabuto prepares the Kisame Edo Tensei, where we can apply the insights gained from yamato's explanation to the Edo Tensei jutsu. Let's see how things go, or Kishi could just reveal little by little as the Edo Tensei bodies fight, if not there's always the databook 4.
EPIC MorrieNoah Jin avy btw.
A bridge towards peace; the will of fire still endures!
Rokudaime Hokage, Rokudaime Rikudou Sennin Uzumaki Naruto
This has probably been asked before, but Im not sure.
Why didnt Nagato bring Yahiko back to life, right after he killed him?
Beware of the Kyubii
Stev3child - The biggest pussy of them all is rokudaime sennin though. If he was a real man, he wouldn't have relied on hax god-given powers to seal the Jyubi. He would have suplexed that bitch and ripped each tail from the beast one by one with his bare hands.
So how do doujutsu users learn that they hold these S-class techniques in their eyes? Like, how did Madara and his bro figure out that they could use Amaterasu, Tsukuyomi, and Susano'o? How did Nagato learn the numerous Rinnegan jutsu?