What it comes down to is that Rex's system needs depth players to have multiple roles. Rex's system relies on a lot of players that interchange a lot. His d will have certain plays where players will only play on that play. In normal d a team could probably get away with have 16-18 d players active on game day, however, Rex will require into the 20s. A guy like Fred who is a backup, who doesn't play D means that is one less player Rex has available for his D. If you are a back up not on the lines, you need to play special teams.