To blur responsibility. Prior seemed to be going for the jugular of the LDS church. For his client, he wants to make the so called "cult" talk mainstream.
I'm not certain that the LDS church shouldn't shoulder a bit this. But I am quite certain that the rank and file of LDS believers are simply being good community members and could easily be offended by that defense. That explains Prior wanting out of Rexburg, but even in Boise everyone is or knows good people who are LDS. So I don't see how Prior's approach of "its not my client, it's your (or your friend's) church!" will work out.
Lori's team is okay with calling the belief system fringe. This makes Lori more a victim of cult- leader Chad.
But really- it's not relevant. If it's not Chad it's his church- still no one told him to kill. If it's not Lori, it's her cult leader, no one told her to be first deputy goddess. And if anyone told her to kill, which is possible, she knew better than to say, "okay, sweetie."
Additional responsible people are simply additional responsible people. They don't make the defendants less responsible.
MOO