I don't see the connection with mIRC being closed source and why you think mIRC scripts should be closed source. They're two entirely separate things.
mIRC scripting pretty much had to be open source when mIRC was first released, if it hadn't been then scripts wouldn't have been used by nearly as many people (for fear of malicious code), the scripts wouldn't have been able to be edited and so wouldn't have been anywhere near as useful. Basically, if mIRC scripts had been closed source mIRC scripting and probably mIRC as a client would never have really gotten off the ground. The points about dangerous code and code editability are still true today.