I know my butterbars are still shiny and new, so disregard this post if I'm totally off-base. I think the point is the whole "force development" concept. It puts proven leaders into new areas, not only to develop the leader, but give new insights into different fields. However, I'd imagine you wouldn't see that happen too much from the non-rated INTO rated fields.
Point in case: My former PAS was an F-16 guy, and on his second assignment as a Colonel, was put in charge of information ops in OIF. He didn't know the first damned thing about IO- but they weren't interested in his expertise, they just wanted his leadership experience.
Then again, I have no idea if there are any proven statistics that say putting rated leaders into non-rated command positions increases mission effectiveness.