Champ Select and Mirror Matches

Forgive me if this has been addressed somewhere before, but is there a specific reason that the new champ select doesn't allow players on opposing teams to select the same champion? It seems like it's either an oversight, or a deliberate choice to ensure pick diversity for spectators. However, it cuts the number of people who can play a champion in a match by half, making it much more difficult to actually test a new champ's kit, or a reworked kit, out against humans.