Does this balsa provide any structural integrity of was it just there as form-work during construction?
Sorry, if this is a stupid question. I'm a civil engineer and not a boat builder, but when constructing buildings and bridges, components are sometimes used and needed during the construction process. Once construction is complete, some form-work cannot be removed due to access issues, but it provides no structural value to the completed project.
Not stupid at all.
The coring is structural in the sense that it is laminated to the inner and outer layers of fiberglass forming a strong but much lighter weight composite. The balsa coring is installed end grain with the grains running from inner skin to outer skin. Although balsa as we know it is soft, it is fairly strong when compressed on the end grain. In addition and a critical point is that the end grain is very absorbent and that allows the resin to penetrate deeply. As long as that penetration is complete, the result is a strong and water resistant hunk of balsa. Unfortunately the layup method is not perfect and sometimes there is a dry spot issue or a separation between the end grain and the outer layers during manufacture that results in the balsa not soaking up as much resin as desired. In addition to resin balsa is also very good at absorbing water. The use of the vacuum bagging method of fabrication reduces the chances of the coring not absorbing resin since the vacuum sucks the resin into the wood pores more effectively that just 'wetting' the ends.
As suggested impacts on the laminated hull can cause small or big separations between the fiberglass and the coring.But it isn't the only cause. It is possible that the incomplete wetting of the core occurred during manufacturing.
When moisture gets (seeps or whatever) between the coring and the inner or outer glass, it will get absorbed easily by and balsa coring that is not entirely sealed with resin.
Wet balsa turns to mush due to a chemical process.
We see that we need at least two factors for the breakdown to occur; Moisture leaking in (think rub rail, gunnel to hull connection, etc and somewhat raw balsa wood.