If it really comes down to this, a couple of things follow. First, Islam could easily embrace insanity (as it has); and second, if God is not linked to reason (He IS reason, of course), then how can any prophet speak for him?
The Pope says in his discourse that the irrationality and arbitrary nature of the Islamic god means that said god could do anything, even command idol worship! In other words, the Islamic god is chaos and nihilism, having no reason or structure within himself or his creation and communicating with human beings solely through arbitrary laws and commands.
It was a great address, not exactly a light read, but worth reading from start to finish. I read it in Spanish, but it may be on the Vatican website in English now.