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.