Well... actually someone alredy took that part of code |V| done and enabled it which causes about this...
1. If the person click to join as observer he'll get to screen like right before start mission and will be able to type message to ppl in game, but wont be able to see anything from inside or see messages from in game pilots.
2. If the person clicks normal join the funny part starts... The poor vicim will completly crash imediatly, if he's lucky he'll get just few error messages if he isn't lucky his PC will completly crush and burn (restart). For ppl in game real fun starts as well... splat of that guy causes typical huge lag spike which can be ok on good hosts, otherwise will lead to chain-reaction of other clients splatting (possibly crushing and burning) which can even cause hosts PC to crash and burn (restart).
...something tells me that |V| knew why it was disabled.
Anyway please could you turn that off again by default? If you want to fix that code please could you put it on custom flag for good hosts only? There is nothing worse than if you can host 2 clients max and right before end of mission someone joins and causes all clients alredy in game to lag-out splat...
P.S. Only way how to defend yourself from in-game joining right now is to host with password, even if the person knows pass it won't let him in.