I prefer using function secureClientCmd_ClientJoin(%client,%num,%ID).
most scripters avoid that command since Badspot decided to mark it with "secure."
It used to be clientCmdClientJoin, but one update it was passively changed to secure. Obviously that means he'd like it to be kept unused.
I'm pretty sure it's just so that the server can't make the client call it, potentially messing something up.If he wanted it unused, he could make it so it can't be packaged, like he has many other functions.