After thinking about this a while (and contradicting my previous reply, though I want to leave it as it is generally correct!) I think this isn't about vapor or condensation at all. I think this is about leaky roofs! If your roof leaks, rain water gets on the wood, and the foam traps it there (even open cell is slow to wick it away) and so it rots the wood away. Worse, since there is no drops inside the homeowner has no warning that things are going wrong and so won't call a roofer to get the issue fixed. (roofers also are not perfect and so if the new roof leaks they won't know to do warranty repairs before the beams fail - in a normal house you notice the drip, and fix the problem before anything bad happens)

The above is my current belief. Who knows if I'm right.