I'm not sure I get why its not allowed, in tourneys and ranked servers sure, but what harm are they doing in say public6?

If they deliberately draw over and over for example that wouldnt be allowed whether it was a multiclient or not. And the argument of "what if its the same person and they want extra goes" becomes a bit moot as well when you consider that its painfully obvious if theyre the same person because they cant type at the same time and it would be really easy to see if they ended up playing against themself