It would make sense to restore more emphasis to levelling your Pokémon up if you wish to use them, rather than just using a few of them to level up the rest. That said, it's not clear if they have the Pokémon at this point to promote sustained engagement with them in that manner. In addition, it might promote more of an approach where Pokémon that are caught are likely to be used immediately, rather than people being able to change their team later on without missing out on a bit, which would probably be what is wanted if they are to be surprised or interested by Pokémon revealed later in the game.
While it is optional, obviously it is included as a part of the game, and its overall design. They are generally expecting people to try and follow the aims set forward in the game, rather than handicap themselves and then try this, apparently.