What I said was basically what I remembered of a naval architects comments. I was not trying to cover all the potential downsides because that's an engineering problem I am not equipt to handle.
It does seem fairly obvious that a strip hull has more rigidity than foam from the word go. We know the strip hull works so it seems logical that we will have to do something to bulk up the foam core hull to be as stiff and to be as capable of warding off penetration.
As far as I can see you have but a few ways of doing that and all of them add up to more weight.
To me the calculus is this. You are saving a little weight with the foam core but are giving up a lot of other engineering properties. If you don't get enough of that stuff back the boat may not have enough margin of safety for all-around use.

: OK, I agree with your points on the material properties of foam, but it's not
: logical to conclude that there is no point to using foam in a boat under
: 25' long. If I decide that penetration is not a big problem, then I can
: get a much lighter hull with foam - granted that it will be more delicate.
: It's also useful for prototyping quickly, has uniform bending properties,
: etc. To make a blanket statement that foam is no good for kayaks is, I
: think, incorrect. Hey, if I make a foam kayak and after a while it suffers
: some penetrations, I can always add extra layers of glass or even
: bulkheads as needed to beef it up, right? Then the additional weight is
: only added where it is actually needed.