I have used the existing proofgrade plywood and I like it a lot. It cuts really well and the pre finish is lovely and its all taped ready to go, but I would not want to use it for every project especially one that needs a little extra strength.
For example the Throwwa desktop trebuchet posted by @cbarker, which I have not cut yet, I want to make in something like baltic birch or a similar plywood that is not fiberboard.
One step further, because its fun, if I want to scale up Throwwa to be built with parts made of 2 layers of Baltic birch laminated together (some re enginering and redrawing would be required) to make a nice small patio model to chuck things into my backyard, I would want the strength of plywood, not fiberboard.
Another small disadvantage to mdf, like a gremlin, don’t get it wet.
While ideas are being thrown into the hopper, what about MDO?
You get the nice strong wood laminate layers of plywood with a MDF like papery top and bottom surface which paints really well like MDF. Great substrate material and its water resistant.