I don't think he'd make a very good soldier, either, but not because he wouldn't be physically competent. I just don't think he's constitutionally suited for it. Harry has always impressed me as being quite sensitive, the partying and clowning around creating a bluff facade for a much softer personality to hide behind. I do think a lot of his antics have been very much a response to trying to find himself in a life where he lost his mother as a young age and is always second to his brother in the spotlight.
William no doubt appears very kind and gentle, but Harry seems even more so, and I think once he grows up a little more and finds his niche (not, I'm afraid, in soldiering), then I think we'll see wonderful things from him indeed.