Co-writing, if done right, means that both parties play to their strengths and ultimately do less work. It's also something that is a common exercise in the non-academic "real world." Most group projects in school do not resemble a work setting in the slightest—you will rarely be called upon to join a group of four colleagues in presenting on something you know very little about. However, you will often have to turn in a final product, written or otherwise, resulting from collaboration and compromise. In fact, many jobs rely almost completely on this model.