[dkarras]

I don't believe it adds any meaningful security that justifies the cost (development, testing, hardening, scaling the state across servers if necessary etc.) With security "more complicated" does not necessarily mean "more secure". Doing it without multiplying the number of ways things can go wrong is deceptively hard.

[shock-value]

With this method there is no additional per-user state, fortunately.

[dkarras]

Yes, just that with regards to security I've seen to many burned by "it can't hurt" processes. With your suggestion, assuming perfect implementation, I personally can't see where it would help. Like, if attacker can run js in your site, they can just set the cookie as necessary before making requests (if the cookie does not exist already) since that is something they can already do. If the cookie exists (most likely scenario), the browser will send it with each request anyways so no added security there either.