That aside, people supporting their own claims with evidence provides a whole host of secondary benefits. It makes it more clear makes it more clear who is claiming what, it pushes back somewhat on the problem of "it takes an order of magnitude more effort to produce bullshit than to refute it", and it prevents some obnoxious debate tactics that reduce discussion quality.
To be honest it don't understand why anyone would advocate any other convention on a discussion board. It seems super short sighted to me.