No DebatArt policy prohibits last-round new argument, but the practice is flawed, particularly for the debate initiator, whether Pro or Con. For the initiator, leaving new argument until the last round is self-limiting, not advantageous, because, while the opponent can still rebut that last-round new argument in their last round, the initiator has no remaining round to defend their last round new argument. One constant is that initiators always have the top frame of each round.
This condition has been attempted to be rectified by calling waived rounds, but that practice is flawed as well, not to mention prohibited, since Policy dictates there shall be argument [including rebuttal and defense] in all rounds designated by the initiator. It has the further imprimatur of cowardice by the initiator by not rendering the first argument in the first round, preferring to see the opponent's argument first. If we initiate a debate, we ought to take the first crack at argument and stop playing waiver games.
We have the opportunity in Description of all debates we initiate to define rules of the road. These are not binding, so say Mods, but, as voters, we can choose to abide by them or ignore them, making it a risk for either opponent to break initiator-defined rules. I have attempted this, myself, and was willing to accept consequences. One rule should always be: No new arguments in the last round, whether or not it becomes policy.