The Defendant is required to appear at the deposition. It may seem redundant given the fact that he has already met with the plaintiff's attorney and signed a "confession," but the deposition may be the only way the plaintiff can preserve his/her evidence in this case. The rules of evidence are complicated, and without seeing the document, I would probably say it is inadmissible. To answer your second question, the advantages of having a defendant attend a deposition are numerous. Not knowing the specific details of your case though makes it hard to discuss the actual purpose of the deposition or its strategic advantages.