You know, I always thought that the 'stupid' Tobi was the Spiral Zetsu 'taking over' as the dominant personality (assuming that he was still some how attached to Obito's body and absorbed so that it didn't look as obvious as when they first merged, or my other theory is that parts of the Spiral Zetsu's body were in fact used to replace the parts of Obito's body that were damaged during his supposed 'death because look here
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the Spiral Zetsu looks as if a part of it's body was cut off but it doesn't really add up because it looks like the right half of the Spiral Zetsu was cut off while the left half of Obito's body is part Zetsu like)
But at the final battle between Sasuke and Deidara , the Spiral Tobi 'dies off' from Deidara's suicide bombing, possibly a significant part of "Tobito's" body got damaged that now Obito was to take over as the Dominant personality, because if you notice it's after this fight that we see "Tobi" being more serious and we also see him talking to Pein and Konan revealing himself as being the sort of "master mind" behind Akatsuki (I wish I could post a link but I don't remember the chapter this happened)
Though maybe that doesn't work either because when Tobi runs into Kakashi's team later, during the final fight between Sasuke and Itachi he acts like "Tobi" despite this being a long time after Deidara's death but this could also be an act...