There are a variety of factors to consider. The basic concern is that it may be unfair to give a debater a chance to give the same rebuttal twice. They may articulate arguments more clearly or efficiently, or make different strategic choices. Additionally, judges have an opportunity to hear the same argument or arguments twice, and so are more likely to follow them. There are reasonable exceptions to this basic logic. When a debater realizes five seconds into a speech that her timer didn’t start, it seems reasonable to simply restart. This is especially true if the speech is a constructive, where the debater is just reading and therefore doesn’t have the opportunity to meaningfully improve her performance.
What is the appropriate way to evaluate close calls? For example, if a debater is twenty or thirty seconds into a speech, is it appropriate to restart it because a timer didn’t start? At Stanford several years ago I encountered a strange situation where a debater started the 1AR before one of the judges on his panel had returned from the bathroom. The judge missed about 20 seconds of the speech, and the debater’s opponent objected to a restart. Should an opponent’s objection be definitive in a situation like this, or should the judge decide whether or not a restart is reasonable given the circumstances?
Some would argue that a restart is never appropriate. Debaters have an obligation to manage their time and equipment like timers and computers, and if something goes wrong they just have to be all that much more efficient with their remaining speech time. Is this unreasonably harsh or the only fair way to handle situations like this?