Competitive people want to compete. Athletics represent competition at it's purist form. Nobody wants a player who is thrilled to sit and watch. That would better describe a fan than a player.
Quality depth on a football team is made up of talented individuals who compete for playing time and in the process drive the starters to perform at the best of their abilities. As the old saying goes, back ups are one play away from being a starter. Therefore, your back up players should have starter qualities, in a perfect world. That statement would be true for every position, with maybe one exception.