I needed a big space for my two kiddos to do art projects. I also needed to get rid of an old dresser we were using to store games, puzzles, and art supplies. The shelves underneath provided the space for all the things previously housed in the dresser and gave my girls 9 feet of glorious work space. Now we don't have to interrupt their creativity to eat dinner.
I modified the parson tower desk by adding a third shelf beneath to hold the longer (9 foot) desktop that I needed. Instead of MDF I used paint grade pine panels. It was only a little more than MDF for the pieces that I needed (1 @ 3/4 x 16 x 96 and 2 @ 3/4 x 16 x 72) and didn't come with the toxic sawdust.