The GPL doesn't force you to release your source if you don't publish the software publicly. Even the AGPL wouldn't force you to release the source in this case, since you're not really allowing people to interface with your software.
MIT-style licensing is the only way to ensure widespread adoption. Companies won't touch GPLed software.
I did realize this is true on both accounts. I'm not by any mean saying switching to GPL is a super great idea I was just wondering what everyone thought about it. As has been pointed out it would do nothing to stop someone from making a private client which is fine. If someone spends the time to optimize a client and use it to there own gain then that fine so long as they don't control the whole generation of coins. I would compare that make your own client to capitalism, where as if we were to force everyone to release everything so the field was completely level would be loosely like communism.