Whether P3P serves a beneficial purpose is a subjective question that is open to debate. My personal opinion is that it probably does not, but others may reasonably disagree.
One of the primary reasons why many sites declare P3P policies is because IE by default will allow third-party cookies for sites that do declare a P3P policy. This leads to problematic incentives, where sites copy-and-paste a P3P policy without understanding what it means. A P3P policy is supposed to be a statement of the site's privacy practices; but often it is not treated that way, it is just copy-pasted to make IE accept third-party cookies. This is an abuse of P3P, but it is widespread. To their credit, Google and Facebook have avoided the temptation to do so. Good for them.
Apart from the legal ramifications, personally I would consider it abusive, improper, and deceptive for a site to declare a P3P policy that it has no intention of complying with. If I discovered that a site I frequented had done so, they would lose my trust, and I would re-consider my relationship with them. In other words, beyond the legal risk, there is also brand/reputational risk associated with violating your own declared P3P policies.