It would be waaaaaaaay easy to complete your Poke'dex.
Its not just because...GAMEFREAK wanted it, it has a reason.
Imagine you had 50 battles in the frontier okay? (Frontier page)
Now in that you will see a: Salamence, Metagross, Garchomp, Infernape and Blastoise. All of these Pokemon are not easy to see, nor easily obtainable in the game. Beacause if you think, you have to evolve the series of all these Pokemon to get the last Pokemon. That will be registered in the Pokedex without any effort.
This is why they haven't included Pokedex registration in the Frontier.