It would appear to depend on how many layers of ply is used. The B&W 800 D3 frame uses 8-13 layers of ply, let alone the glue to put it together. Tonally it may be superior to MDF but then that depends on the speaker drivers you use. MDF will deaden the box tone flat but rely on the drivers.
With ply, you can use it to flex and create better reflections and refractions within the speaker box. I have never done a DIY speaker but I caveat all I have written with, "what I have read in the HiFi and DIY mags".
I'll be interested to see how this thread goes because it is interesting.