MAD publisher Bill Gaines joked that he was a fan of the Fold-In because he knew that serious collectors valued pristine, unfolded copies, and would therefore be inspired to purchase two copies of each issue—one to fold, and another to preserve intact.
My two cents
I always liked the concept of MAD magazine fold-ins. A different way of doing things, and now I realise that it was showing some smart business acumen as well.
Gaines, W. cited in Wikipedia. (2011). Mad Fold-in. Available: http://en.wikipedia.org/wiki/Mad_Fold-in. Last accessed 29th Feb 2012.