>"Full SSL" is still useless against an advanced attacker as it does absolutely nothing to prevent MITM.

I wouldn't say this is entirely useless, even if there is still risk. Someone can impersonate your site to Cloudflare's reverse proxy server if they control the medium between, but they can't just get instant plaintext. This adds an additional barrier for an intruder into Cloudflare's network: they'd have to not only tap all traffic, but inject their own self-signed cert as well. This increases the chances of Cloudflare discovering the intrusion.

I agree though that a pinning solution of some kind would be preferable.