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.
Are there legal ramifications? I don't know. As far as I know, the law is not settled. However, in the US the Federal Trade Commission (FTC) has jurisdiction. They have authority to file suit against any company that engages in "unfair or deceptive trade practices", and they have repeatedly done so. It is generally accepted that if a web site declares a privacy policy, but then fails to comply with their declared privacy policy, then they are engaged in unfair or deceptive trade practices and the FTC has regulatory authority to file suit against the company. The FTC has done so multiple times, and has won. A P3P policy is basically a form of declared privacy policy. Therefore, I believe a company is taking on some legal risk if it declares a P3P policy that it does not comply with. I would not encourage any company to do so, without consulting with legal experts to understand the ramifications.
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.