Im sure its solid when you first build it, lets see how that holds up over the years.
Plywood is simply glued wood dust and compressed fibre with a thin veneer over it. It has nothing to hold the screws/nails in place if you ever move it or it swells or any other thing that might happen over time (especially when you add moisture and such). Its why your IKEA furniture falls apart after you move the first time. Simply adding a bit of solid wood framing means you dont have to worry about all that. It also allows you to square things much easier without the holes like you see in that example above.
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