The first-round-acceptance is there for fairness reasons.
- the rules and notes from the debate initiator take up characters, therefore acceptance-only ensures, that the OP is not handicapping himself from the very beginning
- to get people into the debate without making them read a long set of premises and conclusions to evaluate whether they want to take the challenge
- by not laying open all own arguments before finding an opponent, you don't pre-expose your positions for a group discussion in the comment section
A debate initiator is in the disadvantage from the very beginning. Without a "arguments only" round the opponent already gets one round of rebuttals extra, therefore the fairness-character of a "acceptance only" is very important for at least a bit balancing.
Most people only accept a debate if they know or think they can win. If the instigator posts their first argument in round 1, people may not accept if they think they're going to lose to that argument. At the same time, they will also be able to prepare for their argument (as Theunkown said).
If both debaters accept in round 1, this allows the arguments to be more fair.
In the first round, people usually lay out the rules and everything, it would be unfair to the instigator to have less space to put his/her arguments in the space AND lay out the rules.
So round 1 is made acceptance only so that the instigator has enough space to make opening argument.
Also, its best not to reveal your arguments until someone accepts the debate IMO. If arguments are revealed in round 1, then the contendor can have time to prepare to rebut instigators arguments.
People will only accept debates they know they can win (from looking at instigators arguments), but if the arguments are onyl round 2, then the playing field is level.
What is really the point of just saying that you accepted the debate as the first argument? You know who's accepted by the user displayed. Is there really a point? I say just spit out your first point and get the points coming out on the first round.
Whenever I create a debate I just get right into my points and let the opponent lay down their points in their first argument as well. I won't ever do this as it is wasting time and wasting an argument space. Let more points come between both sides and stop with this "First round: Acceptance" crap.
People must be ready to accept challenges. Entering a debate knowing that you may win takes the joy out of winning. When an instigator posts his arguments instead of acceptance then the contender will have a chance to:
(A) make a rebuttal to that statement
(B) place his arguments
The reason there are multiple rounds in a debate is to ensure that both parties have equal chances of saying why their opponent is wrong as well as giving them the chance of presenting stronger points. Round one is the foundation of the entire debate and if your first words are "acceptance" then the contender will have no idea what the instigator will say. Therefore using the first rounds for arguments is much more fair.