I've also started using water with my shakes rather than milk - not only is casein absorbed more slowly - milk has too many simple carbs for my diet.....just a personal matter really....
I can think of other times that you might want a slower absorption rate though - like when you know that you might have to put off eating for more than two or three hours (horror of horrors!! - but it happens!!).
Really the only time I go for straight whey isolate is right after a workout, or like mickjg mentioned, first thing in the morning....