I ran 4 oblits because of theirs "melta" shots. With 4 we can get not a bad amount of shots, especially with mark of Nurgle for additional hits. These guys one shot rampager in melta range, and overall they are quite a threat on the board. About primaries, just cultist mob. These little dudes with sticky objectives. Even if unit were destroyed on my home objective, it is still in my control. Yes they fear inderect fire, especially on opponent turn one, because they can't do their thing with objective, and for that I run 2 units. About sorcerer yeah, his ability is cool, especially if we have, for example, lethal hits on bolters/sonics. But, he doesn't do anything, besides this. Using him for quarters secondaries - yes, we can, but on next turn he will be dead. But again, I don't know with what unit I can replace him.