It seems that the biggest problem with MDF is it's lack of stiffness and structural integrity. Without pretty extensive bracing, an MDF top will sag given time. A bench top made from SYP or Douglass Fir is pretty comparable in price, and doesn't need the kind of bracing that the MDF top would. Ultimately that makes for a simpler design which is also more flexible to boot. Not that you can't be happy with your MDF bench, that's just the reasoning I used to decide what to make my bench out of...
"History is strewn with the wrecks of nations which have gained a little progressiveness at the cost of a great deal of hard manliness, and have thus prepared themselves for destruction as soon as the movements of the world gave a chance for it." -Walter Bagehot