Ply isn't really the right stuff for longitudinals. It's only half the thickness it looks. 'Douglas Fir' is the 'real' stuff.
Pascoe has always used divinicel, and he uses the harder stuff which supposedly adds some strength, though as he has joins in it (the core), I tend think that's utter bollocks. As Kevin says, the laminate schedule 'over' the former, is far more relevant that the bit of old foam itself.
Douglas fir is hardly heavy, so the real advantage of foam would only really be, that it doesn't rot. But if it aint adding any strength anyway...........go figure