While the contender may have the last word and be able to noob snipe, the instigator has many advantages too. The Instigator can decide the topic, limit who can accept the debate, the time and character limit of the argument, who judges (or who can judge), and also define the words within the topic. The instigator has a 25 point advantage and the contender has a 25 disadvantage, which discourages noob sniping. The instigator can also set upon ridiculous rules, which upon accepting, have to be followed.
For examples, see these debates: