You're absolutely right Bob. MDF swells badly when it gets wet. The trick is to make sure it doesn't get wet by giving is a good coating. I made shutters for my outdoor shed over 4 years ago using MDF. I took care to paint them well and haven't had a single problem yet, and no recoating since. So long as the seams are well sealed, using MDF shouldn't be a problem. I used it because it's (1) cheap, and (2) easy to work with, (probably more stable than ply (i.e., no movement due to changing humidity), and (4) I knew it worked well for outdoor use !
Jim Stewart