I don't think beliefs are really all that designed. Even when we "choose" our beliefs, I think something is going on that is very much unlike design or creation.
I describe belief systems as living systems of information that live along with us. (Do we posses our beliefs? Do they possess us? Or is the relationship somewhat more nuanced than that?) We can interact with the system, and we influence it as it influences us, but sometimes trying to control it and think about your own mind is like trying to run from your own legs or see your own eyes without a reflection.
The human mind is complex. The interactions of beliefs in any belief system are not simple.
The more I muck around with my own beliefs, the more I see it as some kind of organic, self-organizing system with a life of its own. You can influence it, but the process isn't always linear, and what you see or expect isn't always what you get.
Honestly, I find many simplistic approaches to belief change pretty insulting. People just aren't that simple, and neither is life.
Sometimes you want to be as realistic as possible, sometimes you want to fight against what you think is true or see where your imagination can take you. Sometimes your are deluded, other times you're just afraid. Sometimes an unrealistic belief makes you achieve more than others think possible, sometimes it's debilitating.
The mind is not a singular entity, it is plural. We are of many minds and many ways at any given moment. Many systems act in parallel. One set usually obtains, but it obtains only for a time and its rule is colored by the many other activated sets that "lost" but still, most likely, affect the mind and body.
