For an optimum insulin response, protein should be mixed with a high glycemic carbohydrate (i.e. fruit juices, sports drinks, honey, maltodextrin, etc.). Thie idea is that after your work out, your muscles are in prime condition to handle these types of carbs in order to shuttle whatever protein you mix with it into your muscles for restoration.
Though personally I don't really believe in the necessity of having such a PWO meal. Your body is always recovering from the minute you put down your last weight to the next time you have to lift it again. There's a lot of research that supports this. One other note, from a practical standpoint, plenty of people still got in great shape before the whole PWO shake craze. Here is one article I read that helped me come to this conclusion. To reiterate, I don't necessarily think PWO shakes with high glycemic carbs are bad, they just aren't as necessary as some people make them out to be in my opinion.