Venger, that is a good point. I will make an educated guess and say that this personality that Obito/10-Tails have now is a new one. When the 10-tails was called Obito, it was like it didn't know its host name. If Obito was in control with his personality, then hearing one's own name will not be a surprise. So the previous Sage of the Six Paths was even more powerful that we thought because he as the Jinchuuriki was able to keep her personality and still control the power of the 10-tails as there was not talk in the Narutoverse about the 10-tails going on a rampage as we have seen with the other tailed beast!
The true answer to your question will be apparent if Kishi show of where either Obito Jinchuuriki loses control or the 10-tails or where he is having a discussion with the personality of the 10-tails. Also, since this is war, we can assume that Obito is emotionally unstable if the beast form is not manifested, a form that is noted not to be the final form (words spoken by the 8-tails to Killer Bee-san).