Photorespiration is a wasteful process because

Photorespiration is the process by which in the presence of light plant consumes oxygen and releases carbon dioxide (instead of fixing carbon dioxide) during photosynthesis, resulting in a decrease in photosynthetic output since no ATP is produced and carbon (as well as nitrogen in the form of ammonia) is lost inevitably. In this pathway there is no synthesis of sugar, ATP or NADPH, due to which photorespiration refers to as a wasteful process. This process reduces efficiency of photosynthesis in C3 plants.