CStedronsky He would do all of the vocals, but since he also does the screaming it takes an effect on his voice. He wouldn't be able to finish out the set if he did all the vocals because his vocal cords would be shattered essentially halfway through the set. When Oshie or Kameron helps in, it lengthens his vocal cord duration. Many bands do the same thing.
BearButcherBrendan another reason is that singing takes more toll on calendar voice than screening does and it's hard to transition between the two vocal types
Braedon Morrissey I think that's why the aggressive album has majority singing on it. So that he can give his voice a break from screaming so much through the set. Because most songs from the disgusting album are majority screams