Ian, I won't pretend to have any insight. I suspect there's different reasons for each of the clubs to want a second course, but after typing that, the end result, or desired end result would ultimately be the same. Increase revenue stream. Plus, there's probably an economy of scale associated with the costs of maintenance.
At Ballyneal, I know originally the second course was to allow for cart rides, for members (or public play) who might not be able to walk a first or second 18 holes in one day. Plus, with all that great ground, why not use it for it's potentially greatest use? Links golf.
At Prairie Club, their third course is actually the first course, but due to ownership and specific site changes, will be their 3rd. BTW, They are continuing with their public special this year, only at a $50 increase to $399 for one night and two days of golf.
At DR, a 2nd course was always in their plan, too. Originally it was going to be another JN course ( I thought) but then Mr. Watson's showed a great interest and was actually one of the catalysts in convincing Jack to make the changes at DR. Now, with Tiger being brought in, I hold to my feeling that it's a wise business choice, even if from a design stand point, it's an unproven entity. I also suspect that IMG will a major hurdle to overcome for this to ever happen.
Again my speculation....Is by giving the member (and public) more than one course, you can handle a greater number of rounds. Without sacrificing that special feeling of being out with nature with few distractions or other people around.
Mac, Years ago we covered the subject you are having trouble wrapping your head around. It was learned that the name on the shingle gets the credit. Just the way the world works.