I've seen these messages in logs but never frequent enough to cause any impact.

Found it. http://www.securityfocus.com/bid/17958/exploit

The example here shows the pushing of the exploit straight after reading the server version. This is where the client version string is pushed.

http://downloads.securityfocus.com/vulnerabilities/exploits/2680392359-ssh.py

So is blocking the few wild bots looking to this exploit worth while?