There is no real enforcement behind it and it just causes lots of confusion. Seriously I have to go lookup what each of these acronyms are in order to figure out how my privacy is being violated? What guarantees do I even have that you are obeying P3P and not simply sending it to make me feel good.
Hell while we are at it we should implement P3P for phone apps. I'm sure Path (and others) will stop uploading your address book if the P3P says "ADDRBKNOUP"