I understand the need for a hot line and a neutral line in the power system, but in the cord itself? Pointless.
Edit: If the neutral part of the cord is somehow bonded to the chassis, I could see the point then, but then why not just use a three-prong cord and be done with it? Wasn't that part of the impetus for dryers going to a four-wire cord (e.g. separating the neutral and ground prongs)?
3 Prong does provide more protection, but that still doesn't change the fact that if the ground is lost a shock hazard is present.
Also, energized chassis only applies if contact surfaces are metal (such as with my toaster oven). If the chassis of my laptop ever gets energized, I have bigger problems then a faulty ground.
This is why you have polarized and non-polarized plugs, I'll be honest I can't remember the last time I saw anything but polarized. Oh and not all laptop's are made of plastic.