Pokémon - Image #819,840

I thought he gets a new pokedex on each region and does that just to fill it up?

I like what they normally do in these situations. Normally when the come across a Kalos mon Ash scans in, and when they come across a non Kalos mon Serena scans it. That's what they did with the Kanto starters, don't know why they didn't do that here.