First off: Banish is not such an important event. It is one of several events on your daily events list. Perhaps one of the reasons the developers chose to cap the rewards of this event is to keep it from spiraling and giving vastly disproportionate rewards compared to all the other events.

Second: people can test out their formations in action, simply by watching their own fights. In other words, this feature adds no real value.

Third: the more buttons you put on a device, the more newcomers get confused (without a tutorial guide). The only location to put this button (if you REALLY want this thing) would be on the ranking page where you can already practice vs other players.