Flexible SSL is basically equivalent to visiting a totally insecure site over a VPN instead of directly. There's a few advantages to it. Off the top of my head:
1. It protects the privacy of the client still. Nobody can tell which page a given IP address is looking at on a site, since once the traffic is decrypted, it's no longer associated with the originator.
2. Most snooping and MITM attacks happen towards the client end of the connection, which this would protect from.
It's definitely not appropriate for sending sensitive data like credit card numbers, though.
