I've had this thought about GPS (and know I've told people about it), and it's nice to see it validated by a real study. If you use GPS navigation to get from point A to point B there's a really good chance that you will have no idea how you got there. If, however, you look at a map (even mapped directions) and figure out where you are going. You will then have to pay attention on the trip and will have a much better idea of where you are, how you got there, how to get back, and even how to get around problem spots that may occur.
I think GPS is great for navigation if you are someplace you are not likely to return (say on vacation, or a road trip), but it is a much better idea to get around without it as much as possible in the place you live.