Your lucky Jim. MDF, particle board, OSB etc. are all very bad news when moisture is present. Plywood is a hands down safe bet. You're right though, if no water gets to it there is no issue, but for the extra cost, it's not worth the risk.
I just had a dishwasher leak in my house. The underlayment (particle board) swelled up like a sponge and was ruined. The subfloor (plywood) simply needed to be dried out. We use plywood at all our sink countertops (I'm an architect) or at a minimum we use industrial glued particle board.