If it's illegal to send encrypted communications it will still have an impact. If the source of any unapproved encrypted transmissions could be identified there is a good chance the sender could be apprehended.
I guess it will only impact law abiding citizens then.
More generally, the incredible stupidity in those calls for encryption bans is that the "bad guys" will abide to the law. There's a dark market for botnets etc., so why not for custom stealth encryption tools? I wouldn't be surprised if it already exists.