Well, it would give a more solid ground to his argumentation. Neither rationalism nor irrationalism can provide that. He takes things for granted for which his metaphysical and epistemological assumptions cannot account--some would call this "living on borrowed capital." As it stands, he argues very well from the starting point that _humans act_, and deduces from there--but there are important facts to consider that are logically prior to that starting point that should influence his downstream argumentation. You'll likely pick up that I'm referring to the debate between revelation and reason as competing sources of knowledge--when, for my part, I agree with one philosopher who has said "reason is, itself, a revelation."